should i forgive him?

im 27 and hes 34. we been together 6 years. I love him. him cheated on me two years ago. no. im not ok with it. he told me he hasn't done it since an that was the only time he did it. but isn't once a cheater always a cheater? I mean people change and people make mistakes too... I just don't know what to think or do about this. he tells me he loves me but how can u do that if u love someone? we had a falling out when then happened and he says he didn't cheat because we weren't together but I think hes just trying to make an excuse and justify it. but I feel theres no excuse or justification for that. i admit in the beginning of our relationship i wasn't the best girlfriend and i did some stupid things that weren't right but that was the beginning... maybe 1-2 years in certaintly not 4+ years in. i need all the advice i can get. thanks. oh and he told me himself. i had no idea. first he he told me he did it and then he didn't he said it to make me mad and jealous but now hes back saying thast he did... wtf?!?! did he or didn't he? i don't see why someone woul say something like that if it weren't true! what do u think? maybe its stupid that its been 2 years and im still carrying on about this but he said about a year ago it wasn't true and all of a sudden now after we got into an argument hes saying its true... i know when u get mad u say n do things u don't mean a lot... but also when u get mad or drink ur true feelings and the truth has a way of coming out. please help.