It's funny that you are posting this tonight. My mom and I were talking earlier tonight and I made the statement that I did the same stupid thing in both my personal and professional lives: Took too much sh*t and stayed too long!!!
Now have a great job after getting mad enough to leave the other one almost 7 years ago. And my personal life: I don't have one. (Like I said, I stayed too long and it did a lot of damage. I'm probably much too cautious as a result, completely intolerant and thus not seeing anyone after a series of disappointing dating disasters.)
Please don't make that mistake!
If you are really meant to be with this guy -- and forgive me if I seem skeptical that that's the case -- it will happen. Know this though: even if he does come back, you will resent him for this. You may want to forgive and forget, and even think you have, but it will always be between you.
But he is 21, and that is awfully young. Probably much too young to be in a serious, long-term relationship. Let him get a taste of what's out there that he thinks he is missing. As a general rule, guys mature much later than girls -- if they grow up at all. (Unfortunately, I know guys in their 40s, 50s, and even 60s who are less mature than my 16-year-old godson.)
I know it's painful and devastating right now. Rejection is such a vicious thing.
Give yourself some time to heal. Hang out with your friends. Try not to dwell on it. It will be hard, but try. If he is worthy of you, he'll come around.
If not, you are SO much better off without him.