When you wanted sex, you didn't think of the consequences did you!!! When you bite into the forbidden fruit, you have to accept responsibility for your actions. You had no moral qualms about having sex with mum, but now you're trying to wriggle out of a commitment you made the moment you allowed your cock to take charge instead of your brain!!!
How will you get out of your situation? This is fraught with difficulty and risk to you, your mum, and your girlfriend. I'm assuming you haven't told your girlfriend that you are having an incestuous relationship with mum, so that is one threat. You have no job, and nowhere o go if mum chucks you out, that's another threat. If mum is really hurt (and she most definitely is badly hurt), she may feel she has nothing to lose by telling the world about you two, so that's another threat!!! Accepting the worst case, mum may even feel suicidal if you hurt her badly enough, and believe me, she's come to see you as her love in EVERY sense of the word.
Telling her about this girl has hurt her worse than any knife in her heart. I imagine she's now in her forties; her looks are fading, she's probably struggling try to look young, and knows that She's competing with girls that are, literally, young enough to be daughters. Every time mum looks in the mirror, she's looking for grey hairs, lines, sagging skin.... Imagine just for a moment how she feels when you tell her the one thing she's been dreading ever since she first allowed you into her.
Given the way I live my life according to my own conscience, I'm not concerned with the morals of incest, particularly as you've already determined that moral values are immaterial to you (both of you). From a purely objective viewpoint, you have been having sex with a mature woman, which you both encouraged, enjoyed, and took advantage of when you weren't getting sex elsewhere. Now you have a younger model, the old one you want to send to the scrapyard as surplus to requirements.... but it ain't that simple!!!
Get used to the idea that you may need to learn how to juggle!!!