Recently cheated on GF after a few bad months

I've been with my GF for over 2 years and we get on great most of the time and i love her to bits. Recently we've been fighting more often and the fights are getting more intense and alot more serious, we've talked about breaking up but didnt act on it until we broke up for 2 weeks in May, which was her idea. We got back together and things are back to normal most of the time but its like theres this tension beneth the surface ready to explode and i reli dont know whats causing it, nothing has changed on my part and she claims shes the same. I went out last week and kissed someone else, nothing else happened. I think something has changed in me since we've started fighting as i've seen another side to her, its almost like shes trying to get to me or hurt me with the stuff she comes out with in fights and shes almost vindictive when I cant see any reason for it. When we're not fighting she apologies for this and says she doesnt mean it. I just feel like I'm drained and tired of fighting and this has taken its toll on me and made me care less about the relationship, hence kissing someone else, despite the fact I still love her and hate the taught of hurting her. I know I'll get alot of abuse on this thread but im just trying to figure things out Any help or advice would be great!

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