Heart doesn't always get what it wants. ****ty fact of life.
You're not a loser. She's the one losing. She's losing someone who actually cares for her and not just wanting to tap that ass.
One thing I've learned, you can't fix her. No matter how much you care, only one who can fix her is her. Right now, you are just enabling her. You're letting her think her behavior is okay. It's not. Take yourself out of the equation. Tell her how you feel, explain while you're leaving, then cut ties. Maybe someday she'll change, but don't hang your hat on it. Look for someone who deserves you and what you have to offer.