Hey there,
Just wanted to let you know that you're not alone. You and I are in about the same situation. I need to lose 150 lbs though, as I'm normally a 150lb guy, but am currently 300lbs due to a period of bad choices and a recently diagnosed autoimmune disorder. I was 310 at my highest, but have lost 10lbs so far. Starting biking again has really shocked me into reality about how out of shape I am. I thought it would be easy because I used to mountain bike regularly and could handle anything. I also am on Advair and Singulair for my asthma, plus Serevent, Ventolin, and Zyrtec. The funny thing about the autoimmune disorder is that I had been telling the doctors for years that something was wrong because I could barely eat without gaining. It took a lot not to gain weight. One month, I gained 50lbs and hadn't changed my eating habits. Since I didn't go to the doctor regularly, they didn't believe me and just said "Now you're fat, lose the weight and the problems will go away". Thanks doc!
What I'm learning is to listen to my body and stop when I feel like I'm hurting myself. No shame and no worry. I've also started getting off the bike and walking for a while and then getting back on, even though it's embarrassing to be walking a bike. I figure it's still exercise and it gives my asthma a chance to calm down so I can get a second wind. Just stay positive and be proud of your accomplishments, even if they seem small or insignificant. I also am setting really small, doable goals while I ride. Like "I just need to get to the next corner and I can rest." Then I change the goal to something like the next lamppost. And so on. Even though the doctors don't seem to help, keep at it with them too. When they see you are really motivated, it often motivates them to help too. I work at a hospital, so I see it all the time.
Anyway, if you ever just need to vent or talk...I'm around and will be going through the same stuff. Send me a message if you feel like it.
Wishing you the best,
Nico