When I was 18, the girl I had been dating for a year and a half broke up with me and it felt absolutely awful. We did the same things it sounds like you're doing now- took a break for a week or two, broke up, got back together, broke up again. I thought there was no way I'd ever find anyone else who would be as good for me or I would enjoy being with as much as I did with her. It hurt a lot for a few weeks, but eventually I got over it.
Fast forward to now, I'm getting married in just about two weeks to a woman who's 1000x a better fit for me than was the girl I was dating when I was 18. Plus, she's about 100x fitter, has a bigger rack, and is a much better person.
Moral of the story- the girl you're dating at 18 is very, very rarely the right one. If it's not going well, break it off and don't look back.