Replacing the thought "Exercise is Unrewarding"

I perpetually struggle with the motivation to exercise. I’ve tried different activities and whilst some are definitely more enjoyable than others I’ve never really got to the point where it seems fun and I actually want to go. So it requires discipline and willpower to make myself exercise. The problem I have is that exercise feels unrewarding. I feel tired. I lose time I could spend on other things. I don’t see much in the way of results, even when I exercise regularly, and a lapse quickly puts me back at square 1. I’m at my goal weight so not exercising for weight loss but rather health and mental benefits – but I feel better when I’m not exercising! What can I do?