When I fell in love for the first time when I was 18 (he was 7) I didnt know what to do and I went to a priest friend and it was just incredibly lucky that he was not only understanding but he helped me, pretty well single-handed, to come to terms with my being BL. Through him (just through one or two conversations and by the way he treated me afterwards) he helped me to see myself as a normal and good human being. He trusted me completely and it was through him that I decided to go into teaching and give back as much as I could.
It could have gone the other way of course. A lot of priests, even good ones, would have come down on me like a ton of bricks and could easily have messed me up for good. A few years later on, my dad found out through a diary and that was perhaps when I began to get more messed up.
So I did get messed up later, after Father E died, but that was really because I never found anyone else (nor trusted anyone else) to talk to about what was happening in my life and I guess a lot of BLs go through this and for just a few of them it's a disaster because they end up doing bad things.
We don't know either you or your girlfriend so its impossible for us to make a judgement. When I almost had a girlfriend I couldnt cope with it so I told her that I was gay and she hated me for it. I agree with Blackstone that it really depends on whether you are serious in your relationship with your girlfriend but, like him, I would not tell her about being BL but only about being gay and then only to stop the relationship.
Telling her that you are BL would end your relationship anyway and if it didnt, wouldnt it be a very difficult relationship afterwards?
I suspect that you are hoping, like we all do, that this is somehow going to turn out just fine and everyone will get used to it and love you just as much. The truth is not like that but you must search and search for someone that you really CAN trust and I really hope that you can find that person because it can change everything. Just one person to share your secret is all it takes but if you tell the wrong person it can ruin everything. If there is any doubt in your mind, don't tell. When you find the person you can trust you will know. I really wish you well and, although it sounds a cliche, I know what you are going through. Very best wishes, newgeorge