>>2614380>What do we do of the failures of the 20th centuryLearn from them. Above all, reject and strenuously argue against anyone who peddles the view that familiarizing yourself with socialist history or theory is counterproductive or a "waste of time" and that the important thing is that "leftists" all come together and unite to gain relevance, regardless of very real and important differences. Such people, consciously or not, are acting in the service of the same political forces that led to the defeat of the proletarian movement throughout the 20th century and, if you follow their advice, you are doomed to repeat those failures you reference.
Importantly, some of those failures you reference (e.g., police repression and bureaucracy in the communist governments) are legitimate problems and have historical explanations. Others (e.g., the alleged "calculation problem" that bourgeois economists say proves economic planning and the abolition of private property are impossible) are not failures at all but idealist smokescreens that politically act as justifications for counterrevolution. The fact that you lump these all together suggests that you are perhaps theoretically ungrounded. I'd recommend, before anything else, reading Marx and Engels' principle works if you haven't already.