With a chronic illness, the usual approaches to motivate you to exercise simply don't cut it...
Signing up for a fitness class, getting a personal trainer, getting a workout partner are great for a person of normal health, because they provide structure and support - but the rigid schedule is not conducive to a body that is erratic and if you find that you're having to constantly cancel it just adds stress and frustration.
Setting goals is important, but usually that means attaching a timeline to them. But with such unpredictable health, committing to achieve a goal by a given date is often impractical and potentially detrimental due to the added stress.
The usual reasons for the exercise - to drop weight, run an event such as a 10k, look better, etc. can certainly be relevant for some people with chronic illness, depending on your particular condition - but those challenges require consistent effort over an extended period of time, which just seems impossible when you don't even know if you'll feel up to getting off the couch tomorrow...
So what does work? For me, every workout is in and of itself a victory against the illness - even if I've had to miss workouts for weeks - THIS workout was still worth it. I get a sense of empowerment from just showing up at the gym. I have my longer term goals, but I accept that the illness could very well derail them. In a nutshell, you have to find some way to feel that your exercise program is worthwhile - even if that means you can only get in a workout now and then.
Over the years, there have been long periods of time where I wasn't able to exercise more than once every few weeks and it seemed like a waste of time and energy. However, those workouts at least kept me from sliding backwards too fast, keeping me somewhat in the game. Otherwise, I would likely be over 400 lbs now and in a wheelchair, if I was even still here...
I know exercise isn't practical for many, but whatever the case, I just want to encourage you to make the most of whatever you have to work with...