Pessimistic Communism v.s. Pessimism

I remember watching a video in which Zizek was giving a lecture. He was thus challenged by an attending student (I don't remember the following conversations by every single word but this was the idea): "There is no empirical knowledge that socialism had ever been successful, which in turn even caused a great number of tragedies. So why are you (Zizek) still a socialist?"

Zizek first corrected her that he is a communist. His followed arguments were: first, he is never an optimistic communist, that he doesn't really have a solution to the horrible problems these days (at the end of the clip he humorously mentioned "some communists claim that they see light through the dark tunnel. Well I do see light, but it's from just another train coming at us"; second, he believes that major problems like global warming, etc. cannot be solved by the free market or the nation-states (here he added that he was not a populist either, unlike some 20th Century dictators that used popular thoughts and force); and third, China has witnessed a growth whose rate was never achieved by western capitalism - which implies a certain power that capitalism is not capable of (I don't really agree here. For what I've known the economic growth should instead be attributed to the introduction of the market in the 70s). What Zizek did not bring up, at least in the clip I watched, was the theoretical proof that communism is practicable, but I figure that if he could give such proof, he would not call himself "pessimistic" in the first place.

Now, the problem is, assuming that capitalism (or any other form of politico-economic structure), no matter how it evolves, will still have internal, fundamental, self-destructive properties, what is the reason that some like Zizek hold on to pessimistic communism, but not just pessimism? Of course, one can argue that communism has not been given enough chances yet, etc. But given our past experience, will the price not be too high for it to fail again? Zizek himself once quoted Walter Benjamin "behind every fascism there is a failed revolution".

I find a common incompatibility of ideological differences among leftists who often accuse each other as non-leftist -- to me nothing is more horrendous than when one of such incompatible ideologies becomes popular and the majority becomes prompted to suppress and eradicate the minor existences (often then accused as a drag against progress). To a certain extent, market is a mitigated form of this majority suppression in which money as media buffers the huge discrepancy in work values based on popular demands -- I don't see the possibility that this problem can be solved so long as social structure continues existing, though.

I understand that it is hard to embrace life without purpose and meaning. But whenever a meaning or purpose is created, dialectically there has to be some reasonable counteraction with the meaning and we fall into the endless cycle of creating and destroying meanings that don't even necessarily exist in the first place. Does that not lead to simply, pessimism?