There is a surprising amount of snark directed at Texans and Texas on this thread. Also a lack of knowledge about the history of the Civil War and the Founding Principles. It seems that many people have accepted the notion of a massive, debt-ridden, unconstitutional national government in place of a Union of Sovereign States with a weak federal government with very limited powers.
It is sad that a Convention of States has not been done given the overreach of the executive branch, the legislation from the bench by the Supreme Court, and the legislative branch setting themselves above the people they are supposed to serve. We have lost our Republic.
How about we all take a deep breath and drop the insults all the way around. Now to respond to your point. The Constitution was written for the purpose of creating a much stronger national government than existed with the Revolutionary War confederation. Even then with almost no real power within the federal government, states could not leave on their own without approval of Congress and all of the other states. The Constitution established an even more perfect Union with a far stronger national government.
I haven't studied the plan from the secessionists, but one thing stands out. All of the military in Texas have sworn allegiance to the Constitution of the United States, to defend against all enemies foreign and
domestic, and to obey the orders of the president of the US. Upon any formal declaration of secession, the President would immediately federalize the Texas National Guard, thus leading to war within the state itself and Texas having to start from scratch with those few who would commit treason in defense of the movement. Not a good ending.