I have the same issue. I know it's my depression. As soon as my heart rate starts going up, I feel like I'm going to pass out. So, through trial and error, I've learned which exercises I can do for long (like 1/2 hour max) and which ones to forget altogether. Of course, there are days when I can't even walk at normal pace. I have to go really, really slow--like an old lady. But this only happens if I'm trying to get around outdoors and when my depression is really bad.