Hi OP,
There's a lot of abuse on here, and I can totally understand all the negative reactions, obviously. Cheating is an awful thing to do. I've always been so against cheating; I grew up in a house where my Dad had several affairs and in the end my Mum had a bit of a fling with someone else out of hurt I think, and I always vowed that it was something I would NEVER do EVER. But guess what? I did, last summer. & it is the worst thing I have ever done, without a doubt, and I don't think anything will ever top it.
8 months on and I still feel dreadful. It made me feel suicidal. I've messed up at university quite a lot because I've felt so hung up over my actions. Whilst you cannot expect sympathy for this, as I know that is not what you are doing, because you are the one that has commited the "crime", I understand that actually, it is a pretty hard thing to go through. Whilst you have actively made a decision to go and sleep with this guy, this has only happened once at the moment. At this point, you are not having an affair. So what I would say to you is make a decision NOW: do you want to be with the other guy? If so, you have to break it off with your boyfriend. If it is that you just feel like this was a blip and you want to be with your boyfriend, you have another decision to make - tell him or not. Most people would say tell him, but in my experience, if you are genuinely incredibly sorry for it and vow for it never to happen again, I would say don't - it causes so much heartbreak, and if you really feel like the other guy is not what you want then it is not worth causing your boyfriend all that hurt.
What I will say to you is are you sure you're as happy with your boyfriend as you say you are? Usually, it takes something pretty weighty for people to cheat. In my case, things were pretty bad with my boyfriend. I had been besotted with him, and then all of a sudden when he got a new job he changed, he boasted about all the girls at work who fancied him, started doing things to impress them, and then I had a bit of a breakdown during my exam period, and he completely failed to be there for me, even telling me that he was embarassed of me, but that he didn't want to break up with me as I would only keep trying to get back with him and it would be "really ****ing annoying". That completely broke me. I felt angry whenever I thought of it for a good 6 months after, and I couldn't forgive him. Unfortunately, instead of being the strong person I should have been and going on a break or something then, I carried on, putting more into the relationship desperately trying to make it change because I had been so sure about us before, whilst a friend of mine was there for me the whole time, and yeah, the worst moment of my life eventually happened. I was weak, and did the disgusting thing that I will never ever forgive myself for.
After that, I didn't tell my boyfriend what happened, but I told him all the things I was feeling before the incident and said that I wanted a break. I just hate myself for letting it get to the stage that it did before I was strong enough to say what I needed to say - I was so scared of losing him that I didn't dare defend myself, until I did something that makes me no longer ever able to feel like I am worthy of him. I am now thinking about going into counselling as I still detest myself for what happened, and feel like there's so many things muddled in my head. I adore my boyfriend, he means the world to me and we have come a long way in the last 6 months, but I don't know if I will be able to stay with him for ever knowing what I have done and knowing that I don't deserve him. You need to work out whether you'll be able to live with the guilt if you do stay with him and keep it to yourself. People think not telling your partner about these things is the easy way out, but in truth there is no easy way out, admitting to your mistake is selfish in the respect that it makes you feel better, keeping it quiet is selfish in the respect that you're not providing your partner with a piece of honesty - I think the easiest is to cut and run, end things with your boyfriend without telling him about what happened. You just have to make a decision and make it fast; if you can, I would say talk to someone older about it, they usually have the best advice.
I disagree with those who say that if you cheat, you can't love your partner. I certainly still did love my partner, the problem was that there was a lot of confusion and resentment thrown in there that I should have confronted when it first stemmed, rather than ignoring it and hoping it would fix itself. I hate myself for what I did, and for not telling my boyfriend, although I felt it was the best thing to do, and all I can do now is vow to be completely honest about everything that's going on in my head with him from that point on. I still feel like a complete and utter piece of ****, as most people on here will think I am too, but I feel like I have to just be the best person I can be from now on and try and correct my mistake, hopefully eventually being able to put it behind me one day.
One last thing: everyone goes on about the phrase "once a cheater, always a cheater". Try and get that out of your head. It is hard, and you will probably feel like you will never trust yourself not to do this again. But I know I am absolutely determined NEVER to let anything like this happen ever again. You made a massive mistake - all you can do now is to learn from it, and try and live the rest of your life as best as you possibly can, being good to other people at all corners, but to try and look at life realistically, and see potential hazards like this one coming - act to correct them before you fall into them again.
Good luck, and as much as you will, rightfully, hate yourself, try not to be too unforgiving to yourself. Oh, and I know I am going to get seriously negged for this, but the OP wants advice, and while I am completely disgusted with myself and always will be, I wanted to try and give some advice to someone who I felt I might be able to help.