You can't sell a condo you only co-own - all the owners have to agree. And if both of your names are on the mortgage, you can't walk away without seriously damaging your credit.
You can sue your co-owner for 'partition," but the property's upside down, there's not much point in that.
You can also rent out your half of the condo without her consent, although he consent is, as a practical matter required, since it will be hard for you to find a tenant if she's prepared to scare away any prospective tenants.
I'm only licensed in CA. This answer doesn't make me your lawyer, and neither do follow-up comments and/or emails and/or phone calls --- we need an actual agreement confirmed in writing before any attorney-client relationship is formed. Thiss answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue.
If you are co-owners as co-tenants, you have equal rights to use or rent out the condo.
If either of you disagree as to renting out and/or selling, then each of you has the right to file a lawsuit against the other for "partition and sale". This is the remedy when one of the co-owners of real property in California wants to sell, but the other co-owner does not. This lawsuit is a court procedure by which a judge will order the property sold and the proceeds distributed between the co-owners. The downside, however, is that you will not likely get market value for the property. Nevertheless, the filing of a partition lawsuit is often the impetus for the other owner(s) to agree to negotiate a settlement.
In cases where the co-owners cannot agree on the terms of the sale such as the listing agent or initial listing price, the court will appoint a partition referee to carry out the sale. To retain some control, the co-owners could establish parameters which set the bounds of the partition referee’s authority. Absent a consensus, the court simply grants the referee sole discretion and authority regarding the sale. However, any sale will be subject to court approval.
Upon granting judgment partitioning by sale of the property, the court will order that the proceeds of the sale pay the encumbrances thereon, and the net proceeds thereof then be divided between the parties in an equitable amount to be determined by the court, in addition to allowance, accounting, contribution, or other compensatory adjustment among the parties according to principles of equity, pursuant to California Code of Civil Procedure section 872.140. In such a partition action, you will need to present your evidence of why the court ought to distribute the proceeds differently than the ownership percentages.
Note that a partition action does not necessarily mean the property needs to be sold to a third party. One of the co-owners could end up buying out the co-owner(s) and become the sole owner, or alternatively, the court could reallocate the ownership percentages of each co-owner.