For me it took several years. I learned several things about the USSR one by one that softened my opinion of it. I don't even remember what order I learned these things in, but here they are.

• the USSR had a lot of things that the Russian Federation and other post-soviet nations don't (like universal healthcare and free college).
• the USSR's collapse plunged a lot of people into poverty, and the opportunistic privatization that happened in the wake of that plunged even more people into poverty.
• Yeltsin shelled parliament and suppressed popular protests.
• polling data in the USSR showed that most citizens wanted to remain in the USSR at the time of its dissolution.
• the whole "Stalin and Hitler made a pact" mythology that the bourgeoisie like to push is bullshit. The USSR was the last nation, not the first, nor the only, to make a non-aggression pact with Hitler, and they were basically doing it to buy time and move all their critical military infrastructure and factories East to hold off operation barbarossa.
• the Winter War between USSR/Finland only happened because Finland didn't want to let the USSR purchase or trade land with them, so the USSR could better prepare themselves for Nazi Germany's invasion. Even though the USSR was offering more acres of land in exchange than they were hoping to buy from the Finnish in the first place. Also after the Winter War the Finnish immediately became patsies of the Nazis and gave them intelligence.
• the USSR only invaded (a by that point borderline fascist and highly uncooperative) Poland to create a bigger buffer zone between the USSR and the advancing nazis.
• gulags paid their prisoners and were more humane than the American prison system.
• 14 nations intervened in the Russian Civil War on behalf of the White movement, which to me proves the desperation of the international bourgeoisie to prevent the USSR from coming into existence in the first place.
• fascism was basically capitalism's immune system response to the soviet union continuing to exist, and the international bourgeoisie, especially England and America, played a huge role in re-militarizing Germany and helping the nazis during the interwar period.
• the USSR rejected the marshall plan because it would have forced them to privatize their economy and put them in debt to the USA. Not because they wanted to start the cold war for no reason.
• the USSR tried to join NATO in 1954 but wasn't allowed. Yet the year after that, West germany was allowed in, even though they had just got done doing the holocaust 10 years earlier.
• Churchill wanted to immediately re-arm nazis and use them to invade the Soviet union (operation unthinkable) but this idea was scrapped thankfully.
• Operation paperclip and Operation bloodstone are way way way worse than operation Doviakhim, despite people trying to pretend they're mirror images of each other. the USSR kidnapped several hundred low-level German scientists and engineers and used their expertise to help rebuild after WW2. Meanwhile the USA took several high ranking nazi war criminals like Adolf Heusinger and put them in the CIA, NASA, NATO, and the EU commission.
• the KGB never tried to spread socialist revolution around the world. it just kept spontaneously happening in all these different nations in the global south because the working class was fed up. However, the CIA did spread anti-communist counter revolution around the world. Yet the liberals want us to believe that the cold war was two sides, with perfectly symmetrical motivations.
• the USSR lost 20 million people fighting fascism while the USA basically waited as long as they could to enter the war while selling weapons to both sides.
• holodomor genocide is an anticommunist atrocity propaganda created and spread by nazi collaborators. Stalin was literally giving Ukraine and Bengal food aid during the 1930s, ironically when Churchill was doing his best to starve Bengal on purpose.
• if Stalin's homophobia and other problematic aspects of his legacy is supposed to make me ignore everything above and I'm supposed to be anti-communist on that reason alone, then why should I support the liberal democratic party of the USA, which has a record of upholding slavery before and during the civil war?
• way more stuff that isn't coming to mind right now.

It basically was a long process of unlearning a lot of "red fascism" mythology about the Soviet Union. Each time I unlearned something, I was less surprised. I went from going "wow I can't believe I'm defending the USSR against unfair slander." to "of course that was just another lie they told me. why would it have been anything else?"

It's hard to teach other people this stuff, especially because of the Russian/Ukrainian war and all the NATO propaganda surrounding that. Even before then, though, it was an uphill battle. Because people hear their whole life that the USSR was a starving hellhole ruled by despots and torturers and here comes some naive person (or treasonous commie psycho) telling them, no, here's a bunch of facts contrary to everything they know. It just makes me sound like a "Kremlin shill" or something in their minds. After all, how can the experts and reliable sources be wrong? People think that because you can trust the government not to lie to you about how many calories are in a serving of milk, that you can also trust them about state enemies. That's the mistake.