I (20M) have a weird way of masturbating which has fucked up my sex life

So here is some context:

I (20M) started my masturbation career when i was in 6th grade. I did not really have any education on how to do it so I devised my own way of doing it which essentially involves me putting my dick on my stomach and rubbing on it. This is a sound proof way which involves very less motion and gives amazing satisfaction very fast. The problem however is that I am used to this behaviour now.

Coming to current situation:

I have recently bagged an amazing girlfriend (21F) and it is the first sexual experience for both of us. When we first tried having sex neither of us came which is pretty normal. But the problem is that now I am not at all able to finish inside of her (obviously we use protection) and she is very conscious about it and feels that i do not like her which obviously is not the case. I have tried explaining it to her but she still feels insecure about it.

I want to change this behaviour of mine. Even though I can last for very long, my girlfriend does not really want that (yeah idk why). She does not finish either but then I am not worried about that as much as she is worried about my case.

How can I become normal xD? I have tried doing it in the “normal” way but it takes wayyyy too long and it is frustrating. Is this the only way?

ps. I like my girlfriend very much and m pretty sure she is not the reason I do not finish.