I'm in my first serious relationship. How to not fuck it up?

I'm in love for the first time of my life. It's my first girlfriend, the baddest girl I've ever met.

I have quiet BPD and I don't know fucking how, but I manage to not do shit, but I constantly feel like a fraud.

What to do with the feelings of "I fucked it up", the feeling of "I'm fucking this up" and the feeling of "I will fuck this up" that happens to be present 98,8% of the time?