I think that is an underlying emotion for a lot of people that self harm. Sometimes it is also a control thing, oddly enough. For me I think it was far more what you said with a little of what I added thrown in!
I think that there are so many ways to deal with anger and frustration that don't harm other people but honor our own right to feel at the same time. I also think that it makes sense that people don't understand--it's crazy behavior! I cut myself for years but I felt crazy doing it, even though it was a relief and a release in a way. Looking back on it, I realize that much of it centered around not feeling like I could ever reveal my true self, my true feelings, and still be loved. That makes me wonder about the hiding from your girlfriend and at work, etc. That can't be good.
Have you ever talked to anyone about cutting? A therapist or a counselor of any kind? I get that you feel ambivalent about whether it is actually a problem for you are not, so maybe that is not something you want to do right now. I do think you should ask yourself whether it is worth it to try to keep it from your girlfriend?
Good luck figuring it out. IMO self harm, especially cutting is very complex and convoluted and it can take years to understand what it is to you and how to replace it with getting your emotional needs met more consistently.