She cheated on her previous boyfriend and now she cheated on you. She's proven that "it won't happen again" is a lie as she's cheated twice, so it's not a one off thing.
There's no excuse for cheating - if it's because of the drugs and starting uni, then it can be inferred that it will happen if she takes drugs again. Plus if it's because of starting uni then it means that when there's lots of available men around she cheats.
Dump her and find someone worthy of your time. Most people wouldn't cheat, don't let this one experience put you off. Find someone who respects you and doesn't make a habit out of cheating.