How do deal with an ex-girlfriend that wants to be friends?

I have maintained very good friendships with ex's. But we ended our relationship based on mutual reasons. We grew apart, we fell out of love, we were taking one another for granted etc. We decided together that we needed to split and therefor neither was mad or crushed over it. In your case I would NOT remain friends. I've had relationships like this and when you try to go back to being friends it just blows up in your face. And in this case it will be YOUR face because she likes having you around because she knows you're not completely over her. It helps her self esteem and that's just selfish.