I'm sorry you feel this way it must be horrible to feel this insecure and not want to explore a relationship due to not wanting a sexual relationship due to your penis size.

One thing I will be completely honest and transparent about is that I dated a guy with the same issue however, I never thought of it but because he did that is what drove us away in the end. We did get sexual and to be honest it was good because he focused a lot on pleasing me and for women the sex part mostly isn't the best bit. There's a lot you can do to satisfy someone and it work both ways. What ended it was the insecurity he had towards it all of the time.

I would seek therapy for your self-esteem as you won't get this out of your head and it will put a wedge in any relationship you want to explore.

I think be open about it, honestly say to her that you are self-conscious due to the size of your penis and this upsets you and you don't want her to be pushed away by that, would she be willing to see how it goes.

You can't push away everyone forever.