Fast heart rate problem?

I have worked out a year ago for 2 hrs a day, 5 days a week and I could go 30 mins straight on my elliptical. I have started to work out again but I have a problem with my breathing. I start out feeling energized and feeling good. Then every 3 mins I have to stop to catch my breath and I know it has something to do with my heart rate because I feel my heart pounding every time I get out of breath. Then after it slows down I can work out again but it continuously speeds up causing me to burn less calories. Even though I have asthma, I have never had to stop every 3 mins to catch my breath. Is there anyway I can keep my heart rate low while working out? And I have tried to go slower but the restraint also makes my heart speed up.