I have cheated on my boyfriend, help

You need counseling, possibly for substance abuse issues. Seriously - you were awfully drunk and apparently out of control. Being that impaired (even if it doesn't happen too often in your life) is an issue. And maybe that's the thing to talk about with your boyfriend, that you have problems and want to improve yourself. Even if you don't see yourself as being in need of counseling and whatnot, I think you agree this was a bad position to be in. So work on ways to not get into these kinds of positions.

This happened out of town?

Chances are your new BF won't ever find out about it.

Get some feedback from counseling. One question that I would have is how often do you get "really drunk" and how much drinking do you do with your boyfriend? Young people who often get "really drunk" tend to do things that make them ashamed - and that's the beginning of alcohol abuse, since being ashamed is the worst feeling ever, and people drink even more to get away from that feeling.

PS You were very lucky that fella left quickly when you started to say no. You could have been raped!

It was a bit confusing because she said she did have sex with him.

Okay let's face the truth here for a minute.

If you tell him, he might get upset and end things. But you don't want that obviously right? So what are your options? You don't tell him? Is that what you want us to say to you? Forget about it, it was a mistake, don't tell him and just live happily ever after with him?

I know you said not to challenge your statement about loving him, but come on. Drunk, wasted totally blitzed and you easily forget that you love your boyfriend when you are around another guy alone? Not buying it.. sorry, I think you are projecting more than what's really there. Id say honestly that you are completely happy with your boyfriend but at the same time feel shame for wanting something else?

Perhaps I am not a good person to advise on topics of monogamous relationships since I personally believe we are not meant to be monogamous. I really don't see anything wrong with what you did, drunk or not. Really all you are feeling guilty about is the societal stigma about relationships. If you are dating someone seriously then it is wrong to have sex with someone else. I think it is a silly stigma.

I say face it, and roll the dice. Tell him what you told us. See where it goes, at least you can tell yourself that you owned up to it and told him. If it falls apart well just chalk it up to a lesson learned. If he can cope with this information and wants to continue the relationship then it might just bring you closer. Trust always seems to be an issue with relationships so it is always a gamble.

good luck with what ever you end up deciding..

What you are looking for is absolution not advice and you are not going to get it here. Actions have consequences I'm afraid. If not telling him will continue to bother you it WILL negatively affect your relationship with him. Maybe not now, but eventually. So if you do except any advice I would agree that you should tell him. If he really knows you well, and has a dynamic personality he will know if you are sincere and except it for what it was. If he is not, you are better off knowing now so you can move on with out further emotional investment.
You never stated if you have an ongoing physical relationship with your now boyfriend and just how young he is. These factors may heavily influence his reaction to being told. Just remeber, the male ego is a fragile thing. As I said, it takes a real dynamic personality to accept this type of scenaio. If you do get a neagtive reaction the thing you have to think about honestly is how you would react if the tables were turned.
If there is a break up it may not be a permanent thing either. He may after a time decide that you are worth more than his ego and re-establish the relationship.
I would overlook it but then I have been there, done that, got the t-shirt, from both sides. And being older I have learned to control the ego and assess the situation for what it is...hopefully he will do so also.
I wish you the best.