You're using her for your own physical gratification... most women require an emotional connection... physical intimacy is rarely enough.
This is one of the most hurtful things you can do to a person --- give them false hope.
Break it off with her... stop wasting her time. For all you know, she could've been with someone who could've offered her that emotional connection she's been needing, instead of being your 'blow up doll.'
If you want to 'use' someone... save up the money and find a hooker... as least they are willing to participate in being used. This poor girl will now be left to wonder 'what she did wrong' when you break it off with her... and may even have difficulty in future relationships because of it.
"The weakest soul, knowing its own weakness, and believing this truth that strength can only be developed by effort and practice, will, thus believing, at once begin to exert itself, and, adding effort to effort, patience to patience, and strength to strength, will never cease to develop, and will at last grow divinely strong."
- James Allen