Why am I so insecure that I love him even after everything he's done?

You're not insecure, your confused and miss him. Yes, he treated you badly, but you still love him. And, love doesn't fade over night. The best thing you can do is make a list of what horrid things he did to you. Not just the things, but how he made you feel. Remember how lonely you were? Remember the fights? Those are the things you have to remember not just what he did, but how he made you FEEL. Those are horrible feelings. Much worse than the heartache you're feeling now. Keep the list where you can see it daily. Another thing that will make you feel more confident and miss him less is to get into shape. When you exercise, you feel more confident mentally and a good side benefit is you'll look good too.