I am led to step back from the passions of this particular secession crisis and think about the principle at hand.
Imagine that you support secession, in principle. "If a group of people within a specific region of a larger state wants independence, it is their sacred right to pursue and achieve it." Something like that. (Or, more eloquently, "[I]n the Course of human events, it becomes necessary for one people to dissolve the political bands which have connected them with another, and to assume among the powers of the earth, the separate and equal station to which the Laws of Nature and of Nature's God entitle them.")
It is tempting to support this principle. Americans honor the words of 1776, which seem to endorse it, 1861-65 notwithstanding. The problem is, things get very complicated, very quickly. "Where does it end?"
What would be the acceptable number of "nation states" within the territory of today's Scotland?
More plausibly, should Spain split into six minor states, as its various (active) secessionists want?
Here is a map I find at the Daily Mail, a British tabloid newspaper that vociferously opposes Scottish secession. (I haven't looked at all their newspapers, but it seems the Guardian supports Scottish secession, perhaps simply because the ruling SNP is seen as far left; this strikes me as petty.)
Imagine if the current EU, with its 28 states and 500 million people, devolved into, say, 1,000 totally-independent statelets of each around 500,000 people, all under the EU banner and the system more-or-less as it is today. Would this be bad? It would certainly make Europe much weaker. The belief now, of course, especially in Europe, is that we Westerners are past the point of needing a strong state, anyway. What are you, a fascist? Until you do need one. Then "fascists" might come back in vogue. It may, then, be too late. One after another, the statelets of this hypothetical Europe fall, by force of arms or otherwise, to a rising power from the south or east, that doesn't put with all that.