I have serious anxieties about sex, too. I used to be able to take my clothes off and be okay but a partner drew too much attention to the chest tumors (I guess she thought it was funny?

) and now I'm scared of them being touched.
I learned the old school lesbian thing of getting a woman off with my mouth and hands. Most women actually get off clitorally and then penetration is a fun bonus, but your gf could be the kind who gets off from intercourse. It's not that I don't want to engage in intercourse, it's that I get anxious about using a blunt object I don't have a ton of control over (using a harness).
I think being calm and relaxed is the most important thing. Also, if she cares about you, then if it turns out your prosthetic is too small for you, you can always pick out a larger one together.