I'm going through the same problem
I'm only 16, was with my boyfriend for 2 and a half years, we grew up together.
Last week i found out he was hanging around with this other girl..I didn't realise he was cheating on me but had suspisions.
I went over his house to sort it all out & everything was good towards the end of the day. I'd even given him a scrapbook of all our memories. He said he'd stay away from the girl.
She lived next door to him and had just moved there.
Then when i got home i was talking to her & she told me that he'd just been over to hers crying saying that he still wants her, she told me everything, that they had sex twice, had been texting each other, and that he said he was single.
I haven't talked to him for 2 days now (which is good progress seeing as i used to talk to him like every hour), i'm determined to get over him.
Don't get back with the cheaters.
I've even cut my wrists over him. Keep yourself busy.
Things do seem to get harder as the days go by, but everyone says it will eventually get easier, the first few weeks are the hardest when you actually come to terms with the fact he's not in your life anymore.
Good luck, we'll do it together.