I am confused because my feelings seemed to change so quickly

Hi everyone! Thanks for taking the time to read my post.

I am a 28 year-old female who has recently gotten out of a long term relationship with my live-in partner. When our relationship began, it was wonderful. Usually, I seem to doubt my feelings for men once I become involved, but with him, everything was easy and right. However, in the course of the three years that we were together, I began to lose that initial feeling of bliss and became interested in other people - so much so that I would carry on emotional affairs with them for a time, until the guilt became too much and I would put a stop to it.

In the last six months or so, I became infatuated with someone with whom I work. The trouble is, he is only 19 years old. Still, he is smart and witty and fun and, what's more, he made me feel good and alive again, I guess you'd say. My ex was rather inattentive, but this guy seemed terribly interested in everything I had to say. We would email and text each other all the time, and sometimes I would lie so that I could spend time with him.

At any rate, things came to a head and realized that I was over my partner, so we split up. I know this was for the best, although now that he has moved out, I am having a hard time adjusting to the loneliness and the solitary life again. I had really wanted to begin a relationship with the other guy right away because he made me feel so good, and I felt sure of my feelings... However, in the past week or so, something has changed. I no longer feel wonderful and giddy around him as I once did. I seem to only be able to see the things about him that I DON'T like now, as well as our glaring age difference.

I should mention that I have suffered from depression all my life, and was diagnosed with it at the age of 18. I take Effexor and it seems to stabilize me, but I often forget to take it properly or regularly. I've been feeling terribly depressed since the break up, but even moreso, I seem to have become obsessed with the realization that I don't feel anything for this other guy anymore. We did sleep together right after I broke up with my bf, and it's hard to know if I just rushed into something too soon and now my feelings have more or less turned off like a switch, or if this is related to my depression.

I almost want to force myself to feel those things I once did about him. I refuse to believe that I could be so fickle or that my feelings could just change at the drop of a hat. It worries me because this has been a pattern for me in the past, and I don't want to be that kind of person. I am consequently MORE depressed as I realize that this guy is not living up to my "ideal" of what I thought he was, or that he isn't making me feel like he once did. It's not his fault - he's sweet and caring and kind, and I feel like I have used him. Right now, we are just friends, but I seem to want so desparately to get that feeling back.

Do you think it's possible that this is some of depression-related incident, or have my feelings legitimately changed?

Thanks so much.