In order to legally marry someone in the United States, you must first be legally divorced from any previous spouses. If you are not sure whether your Mexican marriage is still valid, you should speak to an attorney to get legal advice.
If you divorce your second spouse but never divorce your Mexican spouse, you may still be considered legally married to your Mexican spouse in the eyes of the Mexican law. This could have implications for your legal status in Mexico, such as your ability to remarry or to inherit property. It is important to consult with an attorney to understand the specific laws that apply to your situation.