Puerto Rico is a territory of the United States. As I understand it, every several years they vote on whether to remain a territory, for statehood or for independence (staying a territory has, of course, always won). How can this nation afford this right to a territory but deny it to a state?
I believe there's even a line near the end of the U.S. Constitution that states if the PEOPLE choose to abolish it, they have that right. What this means is that the rights of the PEOPLE shall not be abridged and it has nothing to do with those who "govern" us. I don't see how, if the people of Texas votes for independence, this right can be denied.
(If anyone has any information regarding these points please feel free to inform me.)
Good luck, Texas!