I also have Ulcerative colitis. I was diagnosed at age 13, and I'll be 23 in January--so it's been almost 10 years for me as well. My UC is pretty mild, and my really bad flares tend to only last a few days at a time when they happen. If I'm having symptoms (bloody stool, mucus, diarrhea, etc.), they usually aren't debilitating and I'm on Asacol to keep myself in control and in remission.
When I was 13, they put me on Prednisone to combat the symptoms and I gained 30 pounds in a month (from 180 to 210), and I was devastated. After I changed doctors and my new one (who said that steroids should be a last resort) took me off them, I went on a diet to lose the 30 pounds I gained. I was still an overweight teenager, but I wasn't an obese one anymore.
I didn't lose weight again until I was 21, and I've lost another 30 pounds--so I'm right at 155 right now. I want to lose about 20 more pounds.
UC hasn't hindered my weight loss, but it hasn't helped it, either. A flare doesn't cause me to lose or gain...it's just one other part of my life that is what it is. I'm sorry that you're on a steroid...all steroids did for me was mask the symptoms and make me gain weight. When I got off of the steroids, the symptoms of UC came back...and I ended up having to control them using other medications (that didn't have all the crappy side effects, by the way). Steroids didn't fix my colitis problem...and unless your symptoms are so bad that you have to be hospitalized during a flare (I have a friend who has had to be), I might consider seeing another specialist for a second opinion about the steroids... There are a lot more options for UC cases than there were 10 years ago.
Good luck with your weight loss goals. UC sucks, it does, but as long as you're determined to do it, it won't stop you from making your goals.
--Kim (Azure)