If you believe you can’t do something then you definitely can’t.
Everyone else has covered the specifics very well but on a general level, you need to rebuild your relationship with your reward mechanisms. It sounds like you’re not allowing yourself to feel satisfied at what you have achieved, because you think it’s not enough. You need to start letting yourself feel good about having done exercise, about getting a summer gig, about being someone who tries, about considering college.
The reason for this is that if your brain doesn’t get rewarded for achieving small things, it doesn’t give you motivation to seek greater rewards, because it starts believing that nothing will bring it satisfaction. I’m talking on a chemical level.
I was in that trap for most of my life and I’ve recently been working myself out of it. It sounds silly but you just have to keep telling yourself stuff like “I worked out today, good job me”. And most importantly: praise yourself for doing the action, not for achieving a result.
I believe in you OP.
I seem to recall some bri'ish guy got done for selling "ied detectors" to the military that were literally a plastic box with an LED light glued on