It's very possible that deep down inside you, you don't feel you deserve her love. Often that can be a big part of holding back when it comes to telling someone how special and beautiful they are. It can give them too much control and add to your own insecurities. Sometimes the goal is to unknowingly bring the other person down to your own insecure level that is what low self esteem can create in a person.

It would be beneficial if you spent time with a therapist so you can figure out where your own low self esteem is coming from.