TBH I agree with you that the invasion wasn't justified. Nor was NATO expansion, orchestrating the 2014 coup in Ukraine, and a whole host of other things. (Disclaimer: I'm an anarchist, so I never see the actions of nation-states as legitimate or justified.)

But that actually doesn't matter. At all. The important thing is to consider what to do now. War is fucking bad. People are dying. The environment is being ripped to shreds, both locally and globally. Capitalists are lapping up profits like nobody's business. Far more important than where some shitty, illegitimate national border winds up ultimately landing is whether the participants in this war keep slaughtering working-class people for their own ends. The most responsibility for that lies with the U.S. and its empire, which is using Ukraine to try to harm Russia, no matter how many lives it has to toss into the meat grinder. It has directly intervened in peace talks and ~~sabotaged~~ overridden ceasefire agreements, and may very well do so again. Of lesser but still high responsibility is Ukraine's government, which was U.S.-installed and has been selling itself, its land (massive privatization to the benefit of U.S. corporations), and its people (conscription, etc.) for the sake of a more privileged position within the empire. And of course Russia shares a lot of responsibility, though getting it to back away from that is hardest because nation-states have very little incentive to resign themselves to existential threats like NATO expansion/encirclement.

So what can you and I do about it? We can pressure the participants. You said in another comment that you live in (or your "home country is"?) Russia. You are in a position to actually pressure Russia to stop invading/expanding and to back away as much as you can possibly make it. You should. Good for you. Many people here are in a better position to pressure the West to do similar: to allow ceasefire negotiations to continue, stop the supply of weapons for war, shrink or dissolve NATO, keep their nation's hands off of Ukraine, etc. We should. Working-class people "on both sides" pressuring the entities they have some small amount of influence on to back down isn't contradictory, but is 100% consistent with socialists fighting the class war. Don't forget that the class war is the only justified "war" there is or can be. As Vijay Prashad has said so well, "War itself is a crime."