According to the supremacy clause of the Constitution (Article VI, Clause 2), U.S. Constitution, Federal Statutes, and U.S. Treaties are "the supreme law of the land" within their sphere of powers, so a state can't simply declare that those laws don't apply to it or within its borders.
Secession would be possible if Congress agrees and enacts a law to that effect (or possibly through a constitutional amendment), but not unilateral secession by one state or group of states without the approval of Congress.
The Republic of Texas was not annexed by Treaty, which it should have been since it was a separate nation. It was accomplished through a vote in Congress and a vote by the Republic, because the two countries could not come to a Treaty agreement. If we voted to be annexed, it seems logical that we could vote to be “de-annexed.” It ought to be simple, since it wasn’t done properly to begin with.
The Constitution is the “supreme law of the land.” That does not affect whether or not one can secede. The states created the union, thus states can withdraw from the union.