I actually posted something similar on another thread. I said "I have low self esteem but I love myself. I have anxiety all the time, I push boyfriends away even if they're great because I fear someone getting too close to me. If I love myself and really like who I am, why do I do this? Why do I have low self esteem and anxiety and push relationships away if I love who I am (which I do)?"
I don't know what causes this. Maybe it has something to do with my self preservation, trying to build a wall before I get hurt or something.
Seems like your situation is slightly different than mine though, since you say you need the reassurance and I tend to need "space" which basically means breaking up with someone out of the blue.
Ugh.
Maybe this isn't a self esteem issue, maybe its just an anxiety issue. Guess we both have to dig deeper.