How to lose weight with my illness

I'm 19 and I have an illness that the doctors can't figure out. When I stand for to long or get my heart beating fast I pass out. So as you can guess I've been gaining a little bit of weight. I've done wii fit and I can do it for about a week before I get so weak I have to stay in bed. So all my efforts go to waste because it takes a week or two for me to feel better. So what my question is what are exercises that work and at the same time I can do them longer then a week and not get stuck in bed.
Updates
+1 y
The reason I want to lose weight is I'm getting married in June and I want to look good on my wedding day.
How to lose weight with my illness
Post Opinion