There are 4 scenarios that could come into play here:
1) you stay with her, you work on the relationship, you try and rebuild the trust you lost, you both learn from this and you're relationship grows stronger.
2) you leave her, you go out and meet new people, you have fun, you do things for yourself, you be a bit selfish, get what you want, try something new and eventually you'll accidentally fall head over heels for some new hottie and then you get an awesome new girlfriend who loves you and is 10 billion times better than the old one!!
3) you leave her, you keep things that remind you of her and constantly punish yourself by making everything about her, you constantly pine for her, make yourself miserable, and constantly wonder what could have been if you had been more forgiving...
4)you stay with her, she does it again, you are so angry and feel even more hurt and used! you're angry for a while and you struggle to sleep and you feel like a complete idiot but suddenly you stop caring because you gave it your best shot and you'll now have piece of mind that you don't have to say what if?
I have personally attempted all of these scenarios with ex boyfriends and in my experience I would say number two is most favourable! go out have a good time meet new people... you are so young did you really think it would last forever? don't take yourself so seriously you sound like a lovely person so demand some respect for yourself by taking control of the situation, ultimately none of us can decide for you and its you who has to live with it?
good luck hope it all works out in your favour!