I need HELP 😞

So my boyfriend and I are fighting again. We have a bad past and he’s messed up a lot especially when it comes to other women. About a month ago he broke up with me and showered with another girl. Said nothing happened. But I’m having a hard time letting it go. He’s going to that town tonight to get a tattoo and stay the night. He told me that this morning and i was instantly upset and just freaked out instead of explaining to him that that makes me uncomfortable and I’d rather he didn’t. That’s my problem I always just freak out right away instead of explaining my anger to him. I keep saying I’ll change and I’ll work on it, but I have such a hard time actually doing that. I need help bettering to express my anger. Does anyone have any advice that I can take?? Because I really don’t want to act this way anymore.