People keep talking about "Federalizing the National Guard" and now you've got other States pledging their NG to Texas in defiance of the Supreme Court (see image).
Most Americans aren't interested or even capable of fighting in a civil war. When you live paycheck-to-paycheck, you're not going to abandon your family to fight on the front lines.
And a huge percentage of Americans live paycheck-to-paycheck.
Bingo. If Texas tried to leave, a HUGE chunk of the population would revolt against the State of Texas. Many more would just leave. Very little good would remain.