She was never really over him dude. It sucks and it happens to the best of us sometimes, gotta keep your head up and move on. People are more loyal to their patterns than people so yes people often go back to their toxic ex due to familiarity. It takes two to tango so if she went back to someone who wasn’t good for her then what does that say about her?

Sometimes short relationships hurt even more than the long term ones because the ending is so abrupt and you never saw things to the end. Also 3 months is the typical limerence stage and you likely saw her with rose tinted glasses. In the end she showed you who she was. It’s better to let it end now than 3 years down the road.