I think you already answered your question about what caused you to develop these patterns. So I will answer a question you didn’t ask — which is how to make change. You are now 26. You’ve spent more than half your life trying to be supportive of your mom. It’s long past time for both you and your mother to shift your relationship from mother-son to adult-adult. Have a loving talk with your mom. Reassure her that you love her and will always be there should she really need you. But you need to turn your primary attention to your girlfriend or you will lose her.
If your mother was happy with her marriage to your father, she will understand that you want no less for your own life. If she was in an unhappy marriage, I hope she will want a better relationship for you. Either way, ask for her blessing and move toward making your own decisions and making your life with your girlfriend your priority.
If you find this too hard to do, you may want to find a therapist who can help you — and maybe your mother — through the process. You really can have your girlfriend and your mother too. Things just have to shift in a few important ways.
I wish you well.