(As a Texan myself)
-The North is richer and more educated and has deeper class structure than the South thanks to the Civil War and past Northern industrialization. I'd say the Northern US is 50-75 years ahead of the South in terms of socioeconomic wealth.
-Nearly 50% of Texas is a mix of Hispanic/Amerindian/Mexican (non Anglo/Germanic European).
-Texas is 12% black. Parts of Houston, Dallas and other cities would simply start to burn once the Federal welfare benefits stopped coming in.
-Many modern "Secessionists" do not understand that Texas's economy is massively dependent on Federal military expenditures and the like. (NASA, US BASES, NAVY, ARMY, AIR FORCE) to name a few. Texas's economy would crash in 10 minutes without it.
-Texas's minority populations would become ungovernable because their Federal welfare benefits would stop. Texas's minority Hispanic/Amerindian populations are heavily located on the Mexican border for hundreds of miles. In many areas of South Texas, we're talking for hundreds of miles, there's less than 10% whites in any of those areas. The rest is 90% Mexican in many of those areas. It's very likely if Texas seceded that much of Southern Texas on the Mexican border would soon be Mexico for all legal intensive purposes or at minimum a "quasi unfunded Mexican state". Texas wouldn't be able to afford to govern those areas because they're completely currently dependent on the Federal government. Texas as a country wouldn't be able to afford to continue to subsidize them as the US Fed does currently.
This is basically what Texas's Southern border would look like (actually this is what Texas currently looks like, imagine if you took away the Mexicans Federal benefits).