I grew up in USSR, it worked fine. The real horrors started after the wonders of markets and capitalism were introduced.

It was socialism and yes it worked tremendously considering the fact that they sacrificed 20M+ lives in WW2 and were subject of western imperialism forcing them to spend most resources on the cold war

My confusion is why collapse back to capitalism?

Let's start with some axioms. There is a continuum of stages of human societal progression with socialism following after capitalism. Under socialism the workers run the state. USSR socialism was great for the worker.

Given the axioms, why would the USSR decide to create a capitalist class again?

The USSR's results in the face of outside pressure were great however. Why buckle after putting up such a fight?

Why can struggle against external power move human societal progression backwards?