I'd suggest dropping her as quickly as you can and jumping back into the dating game 
You're too young to be burning chances at having good sex while in a relationship with a woman who isn't reciprocating your sexual feelings.
Why are you two still together?
It actually does sound like she may be having sex with someone else.
Get away while you can, don't stretch out the torture any longer.

You're too young to be burning chances at having good sex while in a relationship with a woman who isn't reciprocating your sexual feelings.
Why are you two still together?
It actually does sound like she may be having sex with someone else.
Get away while you can, don't stretch out the torture any longer.

or break up with her....
