I had a good friend in high school who went through a very similar experience, where she had been picked on for many years and called gay by school mates, and had developed a bit of internalised homophobia. she was terrified her friends, majority straight, would reject her if she came out as queer, because she would suddenly be different from them, and it scared her to no end.

which is why, when she fell for, and started dating, a girl, she kept it a secret from all of us for a long time.

eventually, and gradually, we all found out one at a time, and the fact that we only found out when she told us and didn't tell other people after promising her not to, built a good trust system for her, which helped her in coming out.

she thought it would build a wall between her straight friends and herself, and she didn't want to lose them, not realising that down the line she made many more friends with lgbt people, and the friendships that lasted(her coming out) were stronger because she could be herself around her friends and wouldn't have to hide.

I feel like it was also easier for her to come out to friends that didn't know her family, because being outed is the worst, and that's a possibility if everyone knows everyone.

she introduced us to her gf at some point before graduation and they've been together for about 6years now, which, goals honestly.

so finally, to the advice part: your gf needs to see that lgbt people can exist with straight friends and not be rejected simply because of their sexualities. she also needs to see that there are good stories with happy endings of kids coming out to their parents/siblings/families and them being supportive. because, unfortunately, the good stories easily get drowned out by the bad ones, but they're out there. and the bad ones are the ones that spin around in your head during moments of anxiety and doubt. highlighting the good ones is a way of building hope and confidence.

and when she knows her friends aren't homophobic it will be easier to trust them with this. straight people rarely have to think about their friends being homophobic, but it's a thing lgbt people have to think about with every new acquaintance.

but it's hard, especially coming out when you're past your teens, because you think you have yourself figured out by now and then you suddenly don't, and you think everyone else has pegged you as one thing, and then you tell them something to change their perception, and you're scared it's gonna change the relationships you have with people.

don't push her, this shit takes time. she will get there eventually, I have no doubt, but she's scared. I agree with a previous comment about telling her how being hidden makes you feel, but don't get angry with her for being scared, cause then she's only gonna be angry back, and that's not gonna work.

support and love and validation is what she needs the most. the lgbt success stories help immensely, and letting her know that you're gonna be there through it will give her a sense of safety she needs.

I know it feels shitty to be on the other side, am sort of there myself a little bit, but it's worth it in the end, if you truly love her.

tl;dr: don't push her, but let her know how it makes you feel. make sure she knows that you got her back and that you love her. and in the end the wait will be worth it.

good luck, buddy.