Sorry for the delay, I’ve been pretty busy and hadn’t got round to answering. I hope it’s not too late for advice?
You only need to come out when you want to, and you only need to come out to who you want to come out to. I’m out to almost all my friends, but not my family. I kidded myself that I wasn’t telling my dad because he would make me tell my mum. They don’t really talk anymore, and I decided to tell him. Nope, chickened out. If you don’t feel ready to tell your mum, you don’t need to. I’m sure your girlfriend will understand.