If you are a lesbian what are the fears making you hide this? Societal pressure, family backlash or something else? I can understand your worry, since people are still not as open towards homosexuality as we would like them to be, but, hiding your feelings isn't going to take the situation away. Will you be happy living your life confirming to what others believe and give up on your chance to be happy with someone of your choice, even if she is of the same sex?
Were these fears also responsible for your inability to relax and enjoy the physical intimacy?
You are attracted but there are many questions playing on your mind. You mentioned the fear of abandonment which might also be playing a role in you not relaxing during intimacy and probably the fear of being accepted as or acknowledging to yourself that you are a lesbian.
I would like to speak with you to understand better your feelings about this situation you are in. So that you overcome these fears and wholly embrace the person you are.
Please feel free to contact me if you wish to speak