Most Helpful Guy
Because being alone sucks ass. When we're lonely we start to long for previous relationships, even if they kinda sucked. You would be surprised how many people are in a relationship just so they aren't alone.
Most Helpful Girl
I know how you feel.
You were attached to him, if you weren't you wouldn't have put up with so much. It's natural to miss him.
But you need to remember that your relationship was very unhealthy and that letting him back in your life would be disastrous. Cut off all contact with him and focus on moving on. You won't feel this way forever.
Wishing you the best of luck.