Hello RRBB342: Welcome to PsychCentral… from the Skeezyks! I hope you find the time you spend here to be of benefit.

I don't know why some people are the way you describe. I certainly always have been. However, in my case, I haven't necessarily always kept my outbursts to myself. Part of the reason I think I handle things the way I do is because of my upbringing. Way back years ago, when I was growing up, I was taught you didn't discuss family business outside of the family. You didn't "wash your dirty laundry in public." And since no one in the family wanted to hear any of it either, I just learned to keep it all to myself. (And there was a bunch of it!) The stress of doing so, however, would not infrequently lead to the types of outbursts you describe.

As a I have written many time before, here on PC, from my perspective stress / anxiety will out one way or another. It's like steam in a closed system. With no other way to escape, it will find the weakest link in the system & blow out there. Perhaps this is what is happening with you?
If underlying stress / anxiety seems to be a part of what's driving your outbursts, getting plenty of physical exercise plus developing a meditation practice may help you to develop more inner peace.

You mentioned you're seeing a therapist. That's certainly a good thing. It is going to be necessary, however, for you to figure out a way to be more open & honest with her. Otherwise there is a limit to how much she can help you. And in terms of how to repair your relationship with your gf, if the two of you cannot resolve this via open communication between the two of you, then couples counseling may be the best solution.

PsychCentral is a great place to get information as well as support for mental health issues. There are many knowledgeable & caring members here. The more you post, & reply to other members’ posts, the more a part of the community you will become. Plus there are social groups you can join & chat rooms where you’ll be able to connect with other PC members in real time (once your first 5 posts have been reviewed & approved.) Lots of great stuff! So please keep posting!