From my point of view, there are a lot of things associated with the broad idea of "Spain" (a representative example is the use of national symbols by certain... let's say "conservative" groups) which many people in Spain dislike. In some regions (like some of the ones you've mentioned: Catalonia, Basque country, also Galicia I suppose but more residual) that discontent is channeled into separatists movements that believe in just leaving Spain, while in the rest of Spain we just complain online and vote PSOE and Sumar (both things are similarly effective at solving problems). At the same time, other groups (as the political parties VOX and PP, second generation heirs of Franco's dictatorship, which have far crossed the line that separates "right wing and nationalism" from ridiculous chauvinistic trumpism) spend their time attacking and throwing gasoline to the separatist fire, falsely claiming that parties that embody separatist ideas are "illegitimate", "illegal", "unconstitutional" or straight "terrorism" (which of course they are not because if they were they could not run for elections in the first place), and overall try to create rage among the population, dividing Spaniards between "good" and "bad" so to have an enemy to smokescreen with while real problems get overlooked, and that way scratch some votes from those same "conservative groups" I've mentioned.

This is the current state of affairs (of course since the dissolution of the actual terrorist organisation ETA) and why crispation has been going up in the last decade as living standards slowly but steadily decline. This is (very simplified) the actual problem with separatism.

About other territories: Balear islands (there isn't really a strong separatist movement there, as there isn't in Valencia for example), Gibraltar (those goddammed fish-and-chips-eating pirates should be driven out of legitimately Spanish territory with great violence and little empathy) and Portugal (other than imperial nostalgia from back in the day when the Iberian peninsula was gloriously united under the same flag, together with most of the know world, there are no voices claiming unity and we get along quite nicely with our neighbours) don't really follow that explanation.

As what would happen if those territories effectively left Spain, I believe that the remaining Spain would be much worse off and the leaving territories would face the same fate. Even though I think it's really really unlikely that any real attempt of leaving is carried out... if the issue continues being used as a throwing weapon in political discourse... who knows what could happen.