Am I a U.S. citizen if my dad was born in Texas but my mom was born in Mexico?

More information is needed for a legal analysis of your citizenship. Here is a good resource for information: https://www.uscis.gov/us-citizenship/citizenship-through-parents. If you were not born in the U.S. but nonetheless qualify for citizenship at birth, it may be wise to apply for a Certificate of Citizenship as evidence of your status as a U.S. Citizen, and it may be wise to consult with an immigration attorney about these issues.