I found out she was sexually assaulted a couple months ago and she had been keeping it to herself this whole time. I was so angry that I couldn't stop shaking and I broke down crying at a bus stop. I let her know that I love her very much and that Im always here to support her. Its obvious she feels at fault but I told her its not her fault but the guy that did that to her. What makes everything worse is that she goes to school in DC and Im in LA. I feel so helpless.

She's a strong girl and had been dealing with issues by herself her whole life. Now that Im in the picture she doesn't have to. I really want to tell her mom so she could deal with it when she comes to visit next week but my girlfriend told me it'll only make things worse because of how her mom is. And I can't convince her to change her mind, she swears its fine as long as she doesn't think about it. But I don't want that guy to get away and do it again. I feel like Im helping him get away by staying silent. She made me promise not to tell her mom and told me she would be bitter at the world if others knew about it.

This is what happened: She had been mixing drinks that night and it messed her up. Her friend, whom she thought of as her "brother" at the time, took advantage of her in a hotel room but she was too drunk to defend herself. She remembers everything. And there was another guy in the room who ignored what was happening. She still sees the person at her school but completely ignores him.

Im not sure what to do... support her and forget it all happened, or tell her mom and risk losing her trust.
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You listen to her when she needs someone to talk to, and encourage her to talk to someone qualified to help. Encourage doesn't mean push, badger, or get mad. Give support, and not advice, would be what I would do. How old is she, and you??

Its fantastic she trusts you enough to tell you.

She will continue to be strong and deal with the emotional turmoil on her own, as Tal says, encourage her WHEN SHE WANTS TO talk about it. Push her to talk about it will have her clam up and stop talking to you altogether.

There is nothing you can do but be there for her when she breaks down. It will happen.

As for telling her mum. If she doesn't want you to, don't. For whatever reason she will when she's ready. It took years for me to tell my mum stuff that I know would have literally killed her inside.

Be the pillar that she'll need. Support her when she calls on you and reassure her that she is strong and not at fault.

Good luck.