You can file a bankruptcy on your own, assuming there are good reasons to do so, and assuming you are eligible for a discharge. You don't say whether you have other debt or who currently has the car.
You should schedule a consultation with a local bankruptcy lawyer who can help you determine whether a bankruptcy makes sense for you.
Who else would you file bankruptcy with? You can only file a joint bankruptcy with a spouse, not a mere friend.
Hope this perspective helps!
Sure, if you file a Ch7 bankruptcy, you must include this debt along with all your other debt. Ms. Bunce is correct you, you and your ex-boyfriend could not file jointly anyway. In addition you will need to qualify for filing a Ch7 by passing the means test. To get answers about your particular situation, I strongly recommend you consult with a local bankruptcy attorney.
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If you are eligible to file bankruptcy, and then you could decide to surrender your interest in the car, thereby discharging that debt. As you can tell by my answer, there are a few "if"s to deal with. You really need to speak to a local attorney in your area to discuss what is best for you.