how can I clean up my credit, got in trouble with joint credit card debt with ex boyfriend

Joint Credit Card Debt: I had a joint credit card account with my ex-boyfriend. When I left the balance was at $0 and I tried to get off of the card but, the credit card company said that I would need him to sign me off. I then asked him if he would sign me off of the card and he refused. I didn't think there was anything I could do and we both had excellent credit. I just found out that last year he wrote a balance transfer check to his mother in the amount of $11,500 and never made any of the payments. Now my credit is horrible (my fico score went from 750 to 450), I was declined a job offer because of it, I can't get approved for a lease for an apartment, and I can't get student loans (or a loan of any kind). The debt is now at $15,000 and I have never made a late payment on anything before. What can I do about this? and How can I repair my credit? - Is this your question? Add additional information