This is a completely normal reaction to a break-up, especially a first-time one. You are still the same person that you were before the break-up! Give it some time; things will start to go back to normal. I know it can be hard.

In the meantime, try to do some things that will boost your self-confidence. Do something that you're good at (sports? writing? gaming? etc). Exercise. Eat well. Do something nice for someone else. Volunteer. Study extra hard and ace that next exam. Spend time with a friend who makes you feel good about yourself. Etc etc.