What you NEED to do, is sit down with this boy and ask him IN PERSON if he has indeed been sleeping with this other girl. If he has, you need to cut ties with both people and you need to work on yourself. What I'm about to say is going to sound cheesey but I've been where you are now. You need to learn to love yourself. You need to find what makes you happy but it has to come from YOU not other people, certainly not boys.
If you break up it won't be the end of the world. You'll need to pick yourself up from your bootstraps and show YOURSELF how strong and awesome you can be. It sounds like you may have a low self esteem and the wrong people will prey on that. Find good people, people who are kind and truly care about you.
Most importantly, take care of yourself. Drink some water take that nap, pamper yourself, have a treat.
You do you boo and the right people and the right boy, will come along.
Take it from a former depressed and suicidal teenager.
I'm now happily married with someone who makes me laugh everyday.