Remember that what you do with your body effects your mind. I see this constantly in myself. If I skip exercise, I start to feel frustrated and antsy. When I keep active I have more energy, stamina and am considerably happier.
But it works the other way too. What you think with your mind effects your body. If you think you can’t do something, you won’t be able to. If you think you can, you probably can. So next time you go to a challenging class, or take a long walk, or whatever you do physically, tell yourself you can.