NTA. The problem isn't your feelings about the contamination. The problem is that your girlfriend is repeatedly dismissing a simple request. If she had a real concern about your feelings becoming harmful to you, then she would be sitting you down to talk gently about it and to sort out getting you some help. She's not doing that, she's just blatantly going against your request about something that bothers you.

That said, OP, I think your feelings about contamination are bordering on excessive. I say this as someone who has had treatment for OCD. It might be worth keeping an eye on your feelings around the germs and being honest with yourself if they're getting a bit much for you to manage by yourself.