Actually I'm not sure it matters that much.

When I'm horny, there are no thoughts other than that I want to fuck. A plane could crash into the building and I wouldn't notice!

That's what you need. Your worries about performance and inexperience have a stranglehold on your perception. You've seen the benefits of changing your behaviour already, all you need now is something to finally tip the scales so that lust takes over.

I think that something for you, is lust. Work on what you want, feeling attracted to the girls, and it will start to drown out the white noise in your head. By which point it'll be on like donkey kong and you can choose to tell her or not and she won't care either way. It may not even occur to you!

You can support this by doing other little things. Watch porn but don't wank. Drink a little to lower inhibition. Lift weights a bit but don't do cardio.