Why am I horrible and mention my bfs ex when drunk?

Anonymous
Me, my sister, her boyfriend and my boyfriend all had some drinks yesterday night in her back garden as the weather was nice. I had too much to drink, and ended up saying horrible things, and mentioned my bfs ex was a druggy. He got upset and walked out, and I didn't see him all night. We spoke the next morning and he said I'm really hard work. I know I'm a horrible drunk, I did tell him before we started drinking, but he insisted on making me drink more.
I dunno why he'd get so annoyed about mentioning his ex, he even said he doesn't have feelings for her, but I'm honestly so confused. Shall I end it with him? Or just forget about it and move on, and jus not drink alcohol around him?
Has this happened to anyone else? How did you resolve the issue?
Why am I horrible and mention my bfs ex when drunk?
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