Im so confused why is he still holding a grudge =(?

ok so me and my ex went out when i first started high school, i was basically forced to go out With him by his cousin, his cousin said "hes a good guy just give him a chance" at the point i was on and of with my other ex and i was just trying to get over him so i gave this guy a chance we went out for a month i think but i cheated on him..... remember [ i was 15] he found out and we stopped talking it never fazed me because i never had feelings for him, 5 years later we meet up again, we hung out a few times and he was all i started to think about, i started catching feelings. by the looks of him i think he did to but he changed alot from 5 years he shows no emotion but u can always tell when someone cares about another, beside the point we where talking for a few months and we fought rapidly one day we got into a serious fight and he said to delete his number..... i did because im not going to keep trying with someone who is hard headed but a few weeks later i spoke to his friend and she said he told her that he was really hurt by me in the past, when i cheated on him 5 years ago but what does that have to do with now.... so he texted me a few days ago and we spoke and squashed any beef we had, yesterday we hung out and the whole time he was flirting with me, even kissed me a few times. but he is still holding a grudge against me i dont get it im soooo confused someone please help me i dont get is i feel like its stupid to be mad at me for what happened when i was 15 im going to be 20 tomorrow..... years past i grew up not so sure he did