Co-signing on any loan is a very dangerous practice.
You can remove yourself from the loan by selling the car and paying off the loan.
If your boyfriend will not agree, then you may have to sue him to force the sale.
You should consult with a local attorney for advice.
This question arises often, which goes to show how much more people really need to understand about co-signing before they sign on the dotted line. It's a legal commitment to the finance company, just as if you'd taken out the loan yourself. You have recourse against your ex-boyfriend, but unless the finance company voluntarily agrees to remove your name from the loan (and why would it, since the very reason they asked for a co-signer was so that they've have someone more reliable to collect from if your then-boyfriend defaulted?), your name will be "off the loan" when the loan is paid off. It could be paid off through refinancing or, as another poster suggested below, you could take legal action to force the sale of the car. You could pay the loan yourself and then take action to collect from your ex-boyfriend or to take possession of car yourself. You could discharge the debt in bankruptcy, but from your description it doesn't sound as if your overall financial situation warrants that. Each of these approaches requires a very specific procedure and may have pros and cons depending on your particular situation. A local attorney will be in a better position to look at the specifics of your situation and explain the processes under your state law so that you can make the best decision. Please talk to one soon--from what you've set forth above it sounds as if your only problem thus far has been with credit reporting, but it is likely that if your ex-boyfriend's delinquencies continue, the finance company will begin collection action against you.
You need to contact the finance company directly and ask them what you need to do. Unfortunately you can not force them to let you off the contract but you may be able to get them to voluntarily agree to it. If not, then you should contact a lawyer and talk about trying to go to court to force your ex to turn over the car so you can sell it yourself. But act quickly because if the loan is very far behind the finance company may repo the car and then they could just sell it at auction and go after you or your ex for the deficiency. Of course, you can talk to them about repo'ing the car and turning it over to you, but that is totally voluntarily for the finance company and they may not agree to it. Call your local attorney Bar Association and ask for a referral to a local attorney right away.