First you should probably decide whether you have a good reason to feel guilty. Are you going against your own moral code? Do you believe it's wrong to have sex prior to marriage?
Perhaps your partner is more into you than you are into her and that makes you feel like you're getting sex from her on false pretenses.
As far as living up to her mother's expectations, she should butt out. It's perfectly appropriate for people your age to explore their sexuality and their feelings for one another.
Freud says there are four people in bed with every couple...their parents. Get them out of there, it's not sexy.
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