My girlfriend cheated on me with my best friend and lied... What do I do about them?

Hi I’m 17 and I’ve had my gf, H, for almost 2 years now. I love her to death, she’s the only girl in the world for me, and I let her know how much I love her every single day. Even so, a week ago it was her birthday and she turned 18. I was at her party, of course, and we had a great time - it was at her place and there were a lot of our friends, but there was NO alcohol. But, the day after, my best friend, P, who I’ve always trusted, told me that after I left, they had sex together. I was heart-broken, literally, I still am. I called and called her, but she wouldn’t answer at first. P apologized to me a million times and said that she seduced him, but I did not believe him. Eventually, she texted me and we saw each other soon afterwards. At first she wanted to know how much I knew, and then she confessed and said she was sorry and that she still loves me the same way and that she doesn’t know why she did it. We talked for hours, and I felt somewhat better, but I was still shocked and felt extremely hurt. I always knew that we are soul mates, and this wasn’t clear to me. Anyway, we went on, and I’ve starting forgiving my friend too, cause he called me every day to check if I’m ok and my mom told me that this probably meant nothing to them and that it’s nothing to worry about. Anyway, I was at the beach yesterday with my cousin and there I saw them- H and P, together there, kissing. Now I’m completely lost. I haven’t told anything about this to any one of them, but I cried that whole day, I’m crying while writing this… Please, what to do? How can this be, what is going on? Should I face my girlfriend and ask her why she did it, or should I pretend that I didn’t see it because she might break up with me?? I don’t know what to do, I still love her with all my heart, but I’m lost?!!