Stop hanging out with this person.
A) Either she will start to miss you terribly and end the relationship she's in to be with you
B) You are torturing yourself by spending time with someone you feel so strongly about but cannot have - don't do that to yourself. If she cares for you at all she will understand why you need to separate yourself from her.
C) It may be a case of wanting what you can't have - she may be less attractive to you once she's available.