I'm male but I know that when I'm attached to someone in a relationship, it can affect our sex life but I think I know why. If one relies on fantasy a lot when you're massturbating, i think it can easily make your expectations in real life higher and that in turn might make you end up dissatisfied. From tehre you can see how you would be less driven to do so. I don't know if that's how you do it - with a lot of fantasizing but it's something to think about.

Another thing is you know exactly how you like it, and have come to the point of being able to fulfill your every need but partners probably are always going to fall short of this.