First you must understand what the Tenth was referring to and what it was not. It was NOT referring to any acts by a state which would affect the structure of the Union. It was referring to those acts of states which ONLY affect the internal aspects of a state. Since secession affecting the entire structure of the Union it is unconstitutional.
How can a State law (secession) overrule the Supreme Law of the Land? Remember that is what the Constitution is and says it is.
The reason the tenth was written was to reassure states that their internal affairs were to remain under their control not a national government's. It was for the most part also a re-assurance that there would be no federal interference with slavery.
Your hypothetical comment on what my Constitution says is incorrect. It is for me what it said it is The Supreme Law of the Land.
The 10th Amendment is in plain english and does not mention the Union at all. Because it does not do this it also gives no power to the Federal Government to force a state to stay in the Union.
That's the entire premise of the 10th Amendment. If the power is not enumerated in it, it does not exist and belongs to the states and to the people.