The North certainly did trade with the South, and thereby make money. But when the Southern economy was cut off from them, they continued to prosper, which the Southern economy ground to a halt.

The Southerners had expected that they had a trump card with cotton exports to the North, but oops.

* * *

Cotton was also a thread (good joke) attaching the South to the UK, and it was where the resentment of protectionist policies came from. It's all related: slavery, cotton exports, non-industrialization, cultural and economic resentments, dependence on free trade rather than protectionism. It was a very bad mix, so it's no surprise that it ended in war.

The timing was such that it was too late for the South to be able to win independence -- the North was already too strong. But when the North was weaker the drive to secession was also weaker because the resentments were bred in part by the stark contrast in prosperity. The North had to get strong enough to win the war for the South to be willing to go or endure the war.

Sam Houston understood all of this, and for his troubles of advising Texas stay in the Union he was removed as governor by the legislature.