I'm not sure exactly how old you are, but I'd guess 20ish. With that being said, yes- you are "too old" do deal with things like this.
But the reality of the matter is that you wrote this other girl an angry note. I understand that what your "boyfriend" did was wrong and of course you're going to be angry & upset about that- that is normal! But personally, I don't think writing a note to the other girl was a good way of handling the situation. It has obviously caused unnecessary drama and it's understandable that you're having a rough time with it.
I'm not sure whether you're still with did boy or not, but my advice would be to get rid of him. Dating a person you cannot trust, regardless of how much you live them, is begging for disaster. Believe me- I've been through a relationship that ended badly like this and I know how much it hurts to let him go, but for your own benefit and health, you have to. I don't care how many times he says it won't happen again or whatever promises he makes you. Don't get back with him and cut all ties with him. Cut all ties with the other girl, too. She isn't worth your time or tears or anger, either.
Distance yourself from the situation and learn from your mistakes. It's okay to be upset, but it's not okay to take that anger out on other people without thinking about the consequences. Everyone makes mistakes- we're only human!- just be careful not to make them worse. I think you should remember that not everyone is out to get you, and whenever you have the urge to act out or speak out when your emotions are running high, try to reign them in and think logically before you do something that you'll regret. I really hope you can resolve this situation as peacefully as possible! Good luck.