Hi There Leah,
Wow, the same thing has happened to me. I think it is a "Lack of the Big Picture" issue. I didn't even think that it might be a part of the Early Onset Alzheimer's raising it's head. I was thinking that it was because of the old saying "If you don't use it, you lose it'. Which I have, lost it that is! I have not had the forced requirement of time management and organization that my stressful job situation placed on me so I lost it. I was diagnosed with EOAD at age 57, January 08 and subsequently was placed on medical retirement. I applied for and recieved SS Disability. I don't have the answers and am still looking. But, I can share what I am doing now. I know you are probably doing much of the same things. I hope that you will have many comments that we can learn from.
I think I found myself so relieved of the heavy burdens of my job that I collapsed into my reliner and let whatever happen happen. But, I also found that I had a lot of frustration when I "went with the flow". I was so often not prepared and even suprised because I did not remember appts etc or I remembered them at a different time or day. So, a while back I decided that it was time to take back my time. I prefer not to go the "electronic gadget way". I feel that my paper solution will stay with me for a longer period of time. Plus, this is the best way for me to immediately reach out an touch. I can flip pages etc with out having to remember how to turn the gadget on and use it. 
I now have a huge calendar (12X15 inches with lots of space to write on the individual day) on the side of my fridge. (Stainless Steel fronts won't allow magnets or I would have it on the front. ) I jot down everything I am going to be doing or need to remember to do. Haircut, Doctors, grocery shopping, when/where/specific hours I volunteer, trash day, clean fridge (that always is on the day before trash day), meet friend for lunch, etc. I do this not only for me but also because I do not always remember to tell my husband what I am doing schedule wise. I will even post things after they happened so I can flip the calendar back and remember specifics. I never tear off previous months. I have been doing this since the beginning of the year. I am always amazed at how many times I look at this calendar EVERY day. I use different color of pens and highlighters. I circle things, make puffy clouds around them. This makes it more "fun" for me to do and catches my eye when reviewing. I was surprised that even routine things that happen each week, like trash and cleaning the fridge out, had to be on the calendar. Otherwise I did not remember. One day seems to runs into another for me. No, it is not pretty but, WOW does it give me peace of mind. I know if I have it written down, I don't have to worry about remembering. Well, except to remember where I wrote it down. 
**One thing that I just realized is that I have a very bad habit of thinking that my schedule and plans are "just not that important any more" because I am not "important" any more. Because I am no longer in the corporate world and mentality. So one thing I need to do right now is to get rid of that "Stinking Thinking". I am important and the things I do are important! **
BTW, I think you were doing a great job just remembering the day and time that you needed to pick up your grand daughter. The only thing I can think of that might have helped is if your husband had also been aware that you needed to pick up your grand daughter. Maybe it would have clicked for him that he would be in the area about that time and could do that for you. This is why I use my calendar. I hope it helps us to adjust to the unexpected.
I also have a small calendar that duplicates the majority of this info. Yes it is extra work but writing it twice helps me remember and I always have my schedule with me. I keep it in my purse. It also has my meds listed and other simple things that I seem to forget. (Phone #'s I don't want to store on my phone, directions to tricky to remember places.) So I always have that with me when I leave the house. I now understand that I do have to plan my time just like I had to when working. I plan errands keeping in mind not only time required, mileage & route, best day & time of day to accomplish looking at the entire day and week. I also keep a small spiral notebook (3X5) that I write everything in. It just flows. I want to go to Hobbly Lobby this week I add it in my book. Over the next few days as I think of things I need there, I will jot them down. The book also has a running grocery list. I may have a Walmart list, phone messages that I want to keep and things I just need to remember. Misc things that I need to do like make hummingbird food and wash bird baths. When ready, I will make my list of scheduled errands in the order I need to run them. I will put any details that I need to remember about the stop. I can't just put "Hobby Lobby" down, I have to put a list of what I need to buy. I also have the coupon for 40% off that I cut out of the Sunday paper in my note book. I have to put a big * COUPON by Hobby Lobby to remember that I have the coupon with me. I also do that with the grocery list. Before I start a new book I go through the old one and write down things I want to remember. The book stays on the kitchen counter, open. It goes with me in my purse when I leave. I am lost if I can't find it.
I think that I wasted so many days, just sitting without a written plan. I could simply not get it together. I find I have to limit many activities that rob my personal productivity. I could sit all day on the computer. Then felt so quilty about it. I now think about the fact that just because I have a progressive disease, I don't have a "Get out of Responsibility Free" Card. I still have those responsibilities. I can't & won't use the Alzheimer's Excuse. I NEED to LIVE MY LIFE and the best way was for me to be in control as much as possible.
Basically Leah, I guess I feel that the realization of the problem helps brings the solution. First step for us is asking for help. We are both there and are looking forward to and in control of the future. I look forward to the comments that you will receive to your post. I know that they will help me to gain even more control of my life.
As Always, Thanks for Sharing. I have used many of the tips and suggestions from your previous posts.
Lynn 
. . . but, I think that you must "forgive yourself" for your inability to be as perfect as you once were so obviously able. Though I do not know the intricacies of your particular diagnosis, it is obvious to me that you are handling it well, and it is a difficult set of circumstances that you now must work within.
I posed the same mental response as that of another respondent: discuss this item with your husband, and reinforce in him how important his help is, to facilitate your ability to be as fully functional a participant as is possible in this new world that has been foisted upon both of you. Also, I would suggest that, in the case of your having the task of picking up a child after school, that the child be instructed in what she should do if she should actually be forgotten. Perhaps you could enlist the eagle eye of her teacher as a backup, to give you a quick call if it is apparent you perhaps have slipped up?
You are doing as well, or even better than most of us would probably be able, in the same scenario. Give youself a pat on the back, and keep up the good work!
been there
You are so right, Sojourner, about the need to instruct my granddaughter what to do in case I don't show up as expected. Thankfully, she is 13 and able to call me if necessary. This new world is quite disconcerting; I know I am too hard on myself at times...I'm working on that!
Many thanks, Sojourner, for your input. God bless you!
Leah,
I live with my Mom who was diagnosed with Early Onset Alzheimer's six years ago. I found that if we kept a calendar on the side of the fridge with everything on it, then a white/dry erase board on the front of the fridge that I updated daily. I would put Today is Monday etc. and list her activities for the day. If there was something that she needed to remember at a specific time I would set the timer on the stove and then put a post it with what she needed to do beside the timer Then when she went to turn off the timer she would see the note.
I hope this helps.
The struggles you have appear to be quite similar to the struggles someone with attention deficet disorder have. I was in an accident years ago and suffered from traumatic brain injury. Organization, short term memory, and time management remain challenges for me.
I subscribe to a magazine called ADDITUDE; although the magazine is geared toward helping those who have ADD I find it to be extremely helpful in providing advice and structure that makes my life more manageable.
You still are the best, Leah. You are judging yourself by past experiences. At one time or another, we've all done exactly what you did. It may be happening to you more now, and that frustrates you since you feel the loss. But your last suggestion, incorporating it into your personality, seems right to me. Be present, be who you are, which is one wonderful woman and teacher to many.
Oh, and don't forget to be human. We all err...
Blessings,
Carol