If being drunk isn't an excuse - it isn't - why even go on and on about it. Oh that's right, you're using it as an excuse, perhaps to rationalise and justify it to yourself because you can't handle realising that you're a bad person.
I don't get the 'honestly I don't remember tag' - I've seen it used by girls before. Sometimes they don't realise you need to be blackout drunk to have memory loss - and have used alcohol when they were tipsy or something to excuse not remembering something. If blackout drunk, as may have been the case here, most of the time the person is so incapable of doing anything they need another person to hold them up properly and aren't capable of sex, though I recognise it is supposed to happen, and isn't an excuse either since you still have control to say yes and no and stop someone.
OP if you read this, just tell your boyfriend. It will hurt and he should dump you. But it is better than him living a horrible lie - he'll have a sack of poo for a best friend, and you for a girlfriend.
People can and do change but I guess you need to break up with the boyfriend, work on yourself and your self control and next time you enter into a relationship put a limit or stop on the alcohol you'll consume, if you want to be monogamous. I still think you should be ashamed of yourself.