My muscles seem to shake when doing sit ups or any kind of physical movement..many other problems (anxiety).?

I think it's due to weak muscles. Don't feel hurt or angry, but a 2 years ago when I was 15, even my hands and legs used to shake while doing some exercise. If it is what i think it is, I'd suggest that you start doing some exercise but only little on the first day, say for 5 min, everyday till the time they don't stop shaking and then slowly extend the time.