How do you have a decent relationship?

I can get girls with no problem, but I suck at actually having a relationship with them. It seems like all I can do is have one night stands.

There was a girl who I did like but struggled to talk to her(for some reason everytime I went to speak to her my mind went blank). Then after 2yrs we became friends and I figured out she liked me also, but I was too afraid to ask her out. Eventually she moved away.

It was weird because we argued a lot but still liked each other. Now that she's gone it seems no girl really measures up to her. She was perfect she had: the eyes I like, the skin tone, the body, the attitude, the hair, she was smart and she let me have my way(most of the time).