Masturbating is fine. There is nothing wrong with it. Some questions for you: Do you prefer masturbation to sex? What is your ratio of masturbation to sex with your girlfriend? Is your girlfriend unavailable for sex that masturbating fills the void? Is it recreational, or release?
[Gf is unavailable in the sense that we see each other on average only three times per week; masturbating does fill that void, but I also just enjoy it for its intrinsic value. I masturbate far more often than I have sex with her.]
Your ability to perform is not the issue. The issue is you questioning your personal sexual needs outside of the relationship. Why do you feel a need to masturbate so much? Is there something going on outside of your relationship that is creating your need to masturbate? Are you masturbating over porn, fantasizing, or another person? The latter becomes a whole other issue. Is monogamy being tested then? See link below.
[I am in the process of exploring my sexual limits but only online; I would never cheat on gf. She is attractive, intelligent, compassionate, steady. But she is also conservative, and I don't know how she'd respond to some of my recent self-discovery.]
http://www.lpsg.org/relationships-discrimination-and-jealousy/24119-what-defines-cheating.html
Would it be fair to say that your need to do this has more importance to you than the relationship? If you have an open monogamous relationship, and you are planning to progress in that relationship together, it might be something that you might want to discuss with her now. Shes going to find out eventually. What will be her reaction be to finding this out? Will it encumber or enhance the relationship?
Again, before your relationship goes farther, this might be something you would want her to know now, before it becomes a problem later. Will she be able to accept and understand your need to do this? If she is adverse to that, both of you have some important decisions to make about the continuance of this relationship.
[You're probably right -- I need to talk more about my sexual complexity and needs with gf. I was being naive to think that I wouldn't.]
Mutual masturbation in a relationship is great if both partners agree to it. The question is whether excessive solo masturbation by one partner is good for the combined relationship? Both partners need to come to an understanding on that point and be comfortable with it. Good luck.