If you care about this girl, defintely dont end it.
Your emotions are probably getting in the way! You mentioned not being fully over an ex.. maybe you have some guilt then about being with someone else now. You could be nervous and anxious about being with someone new. I know I'm always a little more timid in the early sexual stages of a relationship.
This being your first serious lesbian relationship, and that you're inexperienced... maybe you have some anxiety about the whole relationship? Have you been with other girls, or any guys? Maybe since it's new to you, you're just tense, and thinking too much. Try really relaxing and really just going with the flow when you're together. Naked massages always seem to relax me!!
I have to go back for a second to the "should I end it" comment... if you do you still have feelings for someone, and you can't fully give yourself (emotionally) to someone else.. then maybe you do need more time to get over your ex.
If you don't want to give up on this relationship, then don't. The physical problems can be solved. Maybe she needs a new technique, some guidence. Show her what you like, and how you like it. Communication is key in all aspects of a relationship. If you haven't already, maybe you can try introducing some toys.
Hope something I've said may help... good luck!