Why would I say that...

So my boyfriend and I have had our issues. We've had some pretty bad things happen in our relationship but we've gotten through them and I love him more than I love anything in this world. But Saturday I got drunk and I tend to get very mean towards him when I'm drunk. I have no idea why and that's why I don't like drinking around him too much. But this time I was mixing dark and light which was not a good idea. Well today he just got really sad and depressed all of a sudden and kept asking me if I was happy and if I loved him. Which of course I am and I do. But then he started crying and telling me he was sad and is afraid I'm going to leave him and just kept crying. I had no idea where this was coming from. So then I put two and two together and remembered I ended up having a break down when I was drunk about my grandmothers death. So I asked him if I said anything mean to him when I was drunk and he said I did but wouldn't tell me because he didn't know how to tell me. But he said I hit him with everything and hurt him. So I kept asking him and he told me that this past time I was drunk I told him that when we have sex I think of other guys and never if him. Then said the time before that when I was drunk I told him that when I give him head that I think of someone else no think of giving it to someone else and that I have them on social media. I was shocked because I have NEVER NOT ONCE thought of someone else I always think about him and making him feel good! I would never think of anyone else that's just not me. I don't know why I said these things to him. Like I'm crying because I feel like such a terrible person. Why would I do this. And before people try and say he's lying I know this man better than anyone and when he cries over something I know it's really hurting him and that it took everything in him not to break down. Like why do I get this way when I'm drunk. I do like to drink and get drunk once in a while but how do I stop acting so mean towards him when I'm drunk? I truly love him more than anything and want to spend the rest of my life with him so I don't understand why I get so mean... how do I stop it?!

UPDATE: I don't have a problem with drinking I drink maybe twice a month if that. I don't go out of my way to drink constantly because I don't like waiting money on alcohol when I have responsibilities. Just putting this out there.