It sounds like you rationalized cheating because your boyfriend didn't seem to show enough concern for you, but the truth is, you wanted to have sex with another man because you were drinking and felt lustful, right? Because if your boyfriend's behavior really really is so bad that it makes you want to cheat, maybe he's not the man for you. As it stands now, you did something wrong that will probably cost you the relationship if you tell your boyfriend what you did. You will need to decide whether it is better to be honest and lose him, or keep it to yourself and deal with the guilt. Only you know what is right at this point, but you will need to think about either staying in control of your "cheatin" urges while in a relationship (granted that's not always easy, but that is what commitment to another requires - sacrifice) , or else be with men who sleep around too.
I think you already know all of this and just needed a sounding board. No judgement here. Everybody makes mistakes, but you should think about what caused this one so it doesn't happen again.