Frankly, men and women are not that different
The problem is that no one (including me) actually tries to put themselves in the other person’s place when they are “trying” to understand them. We think about what is important to us, and what our motivations and feelings are, and wonder why they can’t accept that as important; but we don’t consider that they have feelings and motivations and things that are important to them.
Put yourself in her place. As I understand it, you are making plans to be with her, then breaking them to do something else. Exactly how would you feel if you and she made plans to go to dinner and she cancelled them at the last minute? Consider that you might have given up a DJ or band gig; that you could have gone out with friends but you had declined knowing that you were going to see someone you loved, made an extra effort to look nice for her, or clean your house?
This, in general, is not a cool thing to do to anyone. When you make plans with someone, they usually are giving something up or making a special effort of some type to be with you. The easiest way to handle this is to not make plans if you keep finding yourself unable to commit. But you may find that you simply don’t have time for a girlfriend at all.
I do tend to agree with your girlfriend that you probably need to “need to stand up for myself, and learn how to say no, and not to volunteer for everything.” Even if it’s the “way you are”. I’ve know many people who do this, explain “It’s the way I am”, and end up being the person that get’s dumped on by everybody because “Twisty will do it”. People will actually lose respect for you when you do this, and they will actually get angry with you, the day that you can’t do what they want you to do, because you have something else that is even more important to you. And you will end up resentful, stressed and angry because you’re putting in all the work, and other people are taking it easy. (They aren’t taking it easy; they stood up for what they thought was fair and they won’t be pushed into being treated unfairly) You should at least not volunteer, or not agree to do something about every 10 or so times, just so they know that you are always available and that it’s none of their g-d business that you aren’t.
People are telling you to talk to your girlfriend, but if you approach it like most people would, you will probably end up without a girlfriend at the end of the talk, and with a broken heart.
You should not go into this expecting that just talking about what is important to you is going to set things right. She is not happy now, and you can’t talk her into being happy.
Your first goal should be to understand what she is losing and what she is going through and how your behavior makes her feel. How does she perceive your motivations? You don’t have to change your life to make her happy, but you must understand why she is unhappy. Then you have a chance at finding a way to make both of you happy. I don’t think you should quit your DJ job; this is a specific, a detail and problems are not in the details, they are in the underlying structure.
You can approach this talk as “You vs. me” or you can approach this as “You and Me vs. the problem” Ask her what you can do to solve the problem; if you don’t like her solution, ask her what she thinks she will get from that and try to come up with a solution that will give her what she wants and still let you be happy.