What did all the countries that tried communism get wrong?

What are you specifically referencing here? No AES state is or has been perfect, that would be a ludicrous assertion. If you specify, we can analyze or fix misconceptions.

I've heard things like it only works on paper and when you try to actually do it humans fuck it all up by being greedy.

This is a misunderstanding of "Human Nature" and Communism itself. First of all, the Mode of Production determines the ideology, laws, etc. Which in turn reinforce the Mode of Production, like this graphic shows:

There is no "Human Nature," greed is more prevalent among Capitalist countries because of Capitalism. Secondly, even if humans are greedy, that doesn't mean Communism doesn't work, that's unexplained.

But for real you think it's possible?

Yes, Socialism has already been implemented and we know it works. Are you referring to upper-stage Communism, ie a Stateless, Classless, Moneyless society? Yes, eventually.

How do we stop the people in power from taking everything for themselves. It's what's happened everytime no?

This is a made-up concern, wealth disparity goes way down in AES countries, like this graphic for the USSR shows:

This is thanks to mechanisms like Recall Elections and other democratization methods, like this model of Soviet Democracy:

I recommend reading Blackshirts and Reds, it's a very good book for dispelling myths around Communism.