You can change how you argue with your mate.
Learning how to argue with one's spouse so that things don't turn ugly isn't something that is innate in people. They are skills that can be learned and it's one of the reasons why we attended the couples workshop, so that we could learn those skills.
It's easy to judge someone on a forum when they only tell one side of the story, but that couples workshop taught me a few things, it forced us to look a lot deeper at ourselves, and what happens during an argument, and why hearing this side of the story doesn't really tell us what happened. It was enlightening.
Let me elaborate a little bit further about "saying something immature" and what we learned at the couples workshop about that.
The workshop was my idea, and TheBF was brave and enthusiastic about going, so when it came time to work on our first "conflict" and "arguing" exercise, I knew that I was going to have to bite the bullet first, to show willing, and to set a good example.
Our first task was to bring up an argument from the past over an issue that is currently resolved but one that got out of control, and try it again, doing it right this time. We had sheets of paper in front of us, with steps instructing us, guiding us on how to do it better this time. It was simple. So we started off on Step 1.
I was nervous, because there have been quite a few times when I haven't behaved well in the heat of the moment. Out of the two of us, I'm more likely to lose my temper, although both of us have had immature moments when we've said or done things that we really don't mean, even said things as immature as the OP's girlfriend. I admit it. We are not perfect, especially when we argue and we lose our tempers. Relationship research shows that behaving badly during an argument is common. If behaving badly couldn't be avoided, then the damage done by that argument needs to be repaired.
And now I would have to face the music first for the first exercise, since this was all my idea. I put on my brave face and was ready to take my lumps.
He brought up one of our earliest incidents when an argument got out of control. I confess, I did not behave well! And as soon as he brought up what I did, I started laughing, out of embarrassment, and he began laughing, too. This was good. We were supposed to be working on maintaining more affection and good humor during our arguments and a willingness to "cut your partner some slack" so the fact that looking back on that incident made both of us laugh now was a very good sign.
This is how he brought it up, "You remember that time you took off your ring (one he bought me) and you threw it at me? What made you do that?" He looked sincere, but a little scolding. At this point, my face turned red, and I began laughing at how terribly I had acted. Mature women are not supposed to throw jewelry to make a point during an argument, and I was embarrassed that I had done so and started laughing out of embarrassment.
I said, "Do we have to do that one? I guess we do. I did that, didn't I?" He nodded his head, eyebrows raised. He had his, "you were very naughty" look on his face. We were both laughing now.
I said, "Well, if I remember correctly... You had just said something extremely mean to me, and I believe that was my retort!" I raised my eyebrows at him. We both started laughing harder.
There was a moment when it dawned on both of us that although I had lost my temper first, TheBF wasn't without blame. He had said something extremely cruel just a moment before, something that he really should not have said and I was understandably hurt, very deeply hurt, and that's when he looked embarrassed. He knew that he shouldn't have said it.
Funny, until that moment, both of us just remembered the dramatic moment of me throwing the jewelry, and neither of us remembered what provoked it until we went over the sequence of events. Considering what he said, my reaction no longer seemed out of place. This was to repeat itself enough times over the weekend that the point was driven home. I had acted out, but I was not the first one who crossed the line. Likewise when he acted out, it was because of something I did or said. Over and over again, even though one person behaved more dramatically or said or did something undeniably immature, we were both culpable because the other person had done or said something that provoked that reaction, something that had caused a lot of pain or hurt feelings, and that meant that neither of us were innocent.
We both realized that the tables had turned. We were both supposed to take responsibility for our part of the argument, concede where we were wrong, and talk about our feelings, and he had been responsible for saying something so very mean.
He said, "Oh... Is that what happened? Because I seem to remember that I just told you that I had rescued some puppies..." And we both laughed at that. Now we both looked embarrassed.
I said, "I see, you thought that I just took off my ring and randomly threw it at you for no reason at all?" He nodded his head hard, pretending that's what really happened, with a wry look on his face. We both laughed.
Then we talked about it for real. We had that argument the way it should have gone, and even though I cried a little, at the end I was so glad that that we went over that issue, with both of us talking about our feelings and why we each said what we said, and it was all done with good humor and affection and understanding for one another, and at the end of it, I felt a million times closer to him than I did before. That happened every single time we had another exercise. There was be these moments of revelation like that, that neither of us were so perfect that we should ever act self-righteous, or harshly pass judgment on the other, moments of understanding about why each of us did or said what we did or said, and closure about so many issues we had from the past, plus all those skills that we learned to prevent future arguments from becoming ugly, and if they did, how to repair the damage so that they didn't continue to be issues in the future, like we did when we were practicing with the argument where I felt so hurt that I threw jewelry at TheBF.
The workshop went splendidly, and we learned a lot about each other and we worked through a lot of things that happened in the past. I can't recommend it highly enough. We've never been closer to one another, and we've been using what we've learned.
Handling conflicts better with your mate is definitely a learn-able skill.