I hear you. I've suffered with allergies (severe) and asthma (fairly mild) all my life. It really can get you down sometimes. Unfortunately, I have to tell you that each of us will have a somewhat different experience, so though we can assume some similarities, it's impossible to predict for someone else exactly what is going to happen from day to day.
Leaving the windows open at this time of year (when pollen levels are often high) can be risky if pollen is one of your allergic triggers. I wish I could tell you otherwise, but I can't. In my opinion, when you have allergies and asthma, living the life you want often comes down to a series of decisions where you weigh pros and cons.
Here's what I mean... I hate air conditioning and love fresh air, so I live with a certain level of allergy symptoms during the summer months that I know I could have avoided if I'd kept my house shut up. At Christmas, I love a real Christmas tree, but again, it does trigger my symptoms. To me, it's worth it, but I'm lucky in that my asthma tends to stay in control if I just take my daily inhaled steroid. The symptoms I'm talking about living with are mostly itchy, watery eyes and sneezing, and occasionally a mild asthma attack.
If you want to be symptom free (or mostly), then you have to do all the things that keep your environment as allergen-free as possible. AND you also need to take your medicine, both for asthma and allergies, if you have them.
Hang in there,