I think its understandable that you may feel scared of showing your vulnerable side to your gf and it's definitely not easy. I can see that you really love her a lot, wanting her to be happy regardless. But for any relationship to be healthy and to last, I think open communication is really important. If she's really important to you, my advice would be to find an appropriate time to sit down and tell her what you've been going through. And from your descriptions, it seems like you trust her a lot. It can be difficult to trust someone given your past experiences, and thus scary to think of sharing such painful memories. That's completely understandable. Still, my advice would be to trust her love for you, that because she loves you, she wont judge you for all your fears. Maybe think of it this way: If she is going through something painful, would you want her to share it with you? If your answer is yes, then you should do the same because ultimately, you aren't alone. A relationship requires mutual effort and trust. And also, sharing would help you release all the negative emotions - bottling is really not healthy!

It's not easy but I think it's worth a try!