My gf is being very insecure?

Honestly, you screwed up with the lies but you probably already know that. You need to show her that you love her and stop trying to explain the ex-girlfriend situation so much because it will only make it worse. You need to SHOW her that you love her not just say it. I know it sounds corny but you need to show a little romance to her. What's also contributing to your girlfriend imagining you with your ex is you being around her so much because it obviously hurts her to see that. Don't feel bad for her and stop trying to cheer her up because she feels betrayed but girls aren't ones for just blurting out what they feel. You need to SHOW her that you've changed and that you're seriously sorry for you lies. Try doing something special for her.