If I co-own a house and want out but my co-owner doesn't want to sell and has been unable to refinance or assume the loan, what options do I have?

In Washington, the typical method to deal with your problem is to file a partition action.  A "referee" is appointed by the court to put the property up for sale or divide the property if that is possible/practical. I assume that you are not a domestic partner with the co-owner, because if you are there are other options. As always, my comments are only applicable to Washington State and are not a substitute for getting competent, local, and more comprehensive, legal help.  

Check with ana ttorney in your area as to the factors involved in bringing an action for partition.  An action for partition asks a court to take an asset that is jointly owned and split it in half.  With money that it easy.  With a house it is not, so a court can order the house sold and the money split.  Some states require that you shouw financial hardship in keeping the property.  Good luck.