“Self-determination” does not necessarily confer to right to split up any political entity. There’s a lot of problems with secession;
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Unopposed secession would inevitably lead to the breakdown of more or less every federal state that allowed it. The instant one state/province/territory wants out, they’ll vote to leave, and then someone else will want out too. The USA had many secessionist movements BEFORE the Civil War, in fact; at one point New England states wanted out.
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Secession never ends. Indeed, West Virginia seceded from Virginia because Virginia seceded. If Quebec secedes from Canada, can northern Quebec or west Montreal secede from Quebec? If not, why not? The populations there would be voting against secession, not for it, so why should the line of secession be drawn at the existing border? You’d have a huge, geographically concentrated contingent of people whose rights as Canadian citizens would be lost against their will. Or if you do redraw the border - well, how’d that work out for the Irish counties that voted to stay in the UK?
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A sovereign nation cannot work if anyone can opt out when they get pissed off. It just can’t; that’s not how democracy works. Allowing secession every time 50.1% of the population of a region doesn’t like it will make effective governance impossible, because you cannot successfully run a country larger than yourself in a way that makes everyone happy all the time.
Self determination doesn’t mean you get to redraw the map. It means you get to have a say.
I’m not saying there aren’t some situations where a people have a legitimate case for independence, but it’s usually when they DON’T havea say, like the people of South Sudan or the Soviet republics or East Timor, etc. etc. And once in awhile you’ll have a friendly breakup, like the Czechs and Slovaks. But pushing for secession from a country you’re a fair partner in is a recipe for more division and pain, not less.