OP, I was going to tell you that you didn't make a mistake, that you willingly went home with someone else and had sex with someone else. That having a few drinks doesn't absolve you of of responsibility of your own actions. I know this, because I've done the same thing after a few too many beers.
But then I read your follow up posts, and none of that is true. You didn't make any decisions because you weren't in any fit state to make those decisions. Someone sober took advantage of the fact that you were falling down drunk, that you couldn't consent, and raped you.
You didn't make a mistake, you didn't make a decision. You were taken advantage of by someone who didn't have your best interests at heart.
Plenty of people will tell you that you need to tell the police, or that you need to tell your boyfriend. You don't have to do either, that decision is up to you.
I wouldn't recommend the former, I saw what my sister in law went through, she found it almost as hard as the sexual assault, and it was all pointless as she didn't get a conviction.
I would recommend the latter. Trying to keep this a secret is not going to help your mental health. Your boyfriend might take it well, he might not. Either you get support or he splits up with you, but either option is likely to be better for you that trying to keep it hidden.
And if you do tell him, you should be telling him that you were raped, not that you cheated on him.