Some things that moved me from squishy western "leftism" into actual leftism:

• The financial crash of 2008. All the adults in the room were showing their entire ass and the only people giving an explanation of what was happening that actually made sense were self-declared Marxists like Richard Wolff
• Local political developments made me realise the futility of the soft squishy boyscouts passing for acceptable leftism around here. For the past 30 years we have had increasing racism, greater divide between rich and poor and an expansion of the police state. The "left" around here has shown itself to be completely unable to do anything to stop or delay these developments. At festive occasions they stand up and say "how dare you!" and then they go on to be nice and friendly to the ghouls responsible.
• Experiencing actually existing social democracy in action. If you believe the fairy tales they tell you in social studies class, getting a succdem-controlled government should be an improvement over a liberal/conservative one. They might not exactly create a worker's utopia but at least they might make some improvements here and there and be a little less evil. After all they depend on the "left" for parliamentary support, right? Nope. The Iron Law of Late Capitalist Electoralism is true, every administration, regardless of party affiliation or electoral primises, is going to be worse than the preceding one. The succdems are just as nasty as anyone else and the left will do nothing to rein them in.
• Becoming aware of climate change, the monumental scale of the task ahead and seeing how ridiculously small and insufficient policies are being touted as the solution by the establishment.
• Learning about history, especially the history of the USSR, WWII and western imperialism. There are so many omissions, distortions and outright lies in what you pick up from the western education system and popular culture that you begin questioning all of what you've learned once you start learning from more nuanced sources.

Some sources that opened my eyes along the way.

• My mom. I know it is cheesy and not applicable on a large scale but she deserves a lot of credit for laying the groundwork for my political development. She used to be a member of the communist party in her youth and although I never felt her trying to force any beliefs on me, but she presented me for an openness to communism and anti-capitalist ideas and introduced me to communist culture like the collages of John Heartfield and the novels of Hans Scherfig.
• My dad. Again a cheesy choice. Today the man is a deranged tradcath nerd-chud but credit where credit is due, although insane he is an intelligent and educated guy and his intellectual snobbery meant that I learned about stuff like formal logic and critical thinking from an early age which later on made it easier for me to break out of the liberal mind-prison.
• Chomsky. The guy is problematic but his propaganda theory and his work on US imperialism did a lot to move me left back in the day.
• Richard Wolff. He might be a bit too preoccupied with forming cooperatives under capitalism but he did a lot to explain how the capitalist economy works.
• Parenti. The guy is a legend. You might not be receptive to him if you are still full of anticommunist brainworms but once cracks have formed in that mindset he can expand those cracks quickly.
• Graeber. His intellectual originality really helped to question a lot of the preconceived notions about debt and money that supports capitalism.
• On a more local level I want to point out two people who helped me understand Danish history: The first one is Carl Heinrich Petersen who was an anarchist historian who wrote about the labour movement to the left of the succdems. I credit him with puncturing some of the common rose-tinted myths about inter-war succdem leader Thorvald Stauning. Then there is the before-mentioned author Hans Scherfig whose novels Idealister set in the inter-war period and Frydenholm set during Nazi occupation gives a good understanding of the mindset of nazi collaboration, the fecklessness of non-materialism and liberalism and the memory-holing of communist resistance ans well as bourgeois collaboration after the war. Another Danish author of note is Carl Madsen, a communist lawyer who wrote about the class nature of the legal system, official collaboration with Nazi Germany before and during WWII and pre-war refugee policy. He also wrote some pieces explaining the Democratic German and Soviet legal system from a positive standpoint.