I have a really hard time cunning with my girlfriend?

It's tougher for some guys than others, for a variety of reasons. You say she's the only one who's been able to get you there, which implies that you've tried with other girls before. Sometimes the reason is emotional--as you become emotionally closer to her, you'll climax more and more easily. Are your feelings for her perhaps stronger than you've had for some other girls? Also, depending on your masturbation habits, you could be desensitizing yourself; I would advise you to stop altogether and see if that helps. It may take time. Keep at it--generally speaking, your sex life will improve the more time you've spent with your partner. After a year with my fiancee, she's able to do things to me no one else could ever come close to!