I finished with 2 blokes when I was 17-18 ish. For what some may class as a trivial reason.
The first one, I had been dating for about 2 to 2.5 months. (I was 18, he was 20.) I was getting a bit fed up of him anyway, as he was flaky. Stood me up several times, when we arranged to meet in the pub, sometimes met me in a dark mood, (hardly speaking to me and spending more time on the pinball machine,) and also had occasions where he would walk off and leave me for 20 to 30 minutes (when we were in a nightclub,) to chat to his mates. And no, even to this day I don't know why I tolerated it.
Anyway, so he came to my house to get me so we could walk to the pub together, and I said I am nearly ready, I'll be five minutes. He rolled his eyes and sighed, and sat down. I was playing 1950s music on the record player.. (this was the late 1980s,) and he said 'turn that shit off.'
I said 'but why? It's good music.' 'TURN IT OFF' he demanded! With a face like this > 
I said 'no.' He said 'if you don't, I'm leaving, and we're done. You're dumped if you don't turn that fucking shit off.' I turned it up and said 'see ya then.' He stormed out. I felt such a surge of relief go through me. I'd got rid, after 2 and a half months
About a week later, he rang (my mum answered) and he asked for me, and said 'what time does she want me to come pick her up to go to the fifteen bells?' (this was the pub...) Fucking cheeky bastard. 
I had told my mum everything and how relieved I was to be free of him. She told him to piss off and slammed the phone down on him.
Never heard from him again. Result!
The second one was when I was 17, and I had lost 2.5 stone after being a bit of a chubby girl in my early teens. I dropped from 12 stone to 9 and a half stone. (I am 5 ft 5.) I felt great, I looked better, and loads of people said that I looked so much better, and I felt more confident.
I had been dating this lad (aged 18,) for about 4-5 weeks, and we sat together on the couch in our dining room, and he squeezed me around the waist, and said' got a little bit of flab there.. YOU can pinch more than an inch eh?' with a stupid grin on his face.
I felt myself go flame red in the face with embarrassment and rage. I stood up and said 'get out!' He looked gobsmacked, and said 'whaaaa?' I said 'I have spent a YEAR losing 2 and a half stone, and I am proud of what I've done. I don't need someone in my life, who is telling me I am still fucking fat!' He said 'oooh you're a bit touchy.'
I SHOVED him out of the house, and refused to take any of his calls, or ever come to the door to him again. He tried for about a MONTH to get me to go back with him, but I refused. Again, I wasn't mad keen on him anyway, so it was a good excuse to dump him.