I think what we have to learn to let go of are our opinions. Once you have an opinion of something your mind closes up. And it's probably pretty accurate to say that we really aren't smart enough to ever really know the truth about much of anything. The universe is too big and we are too small.
But we are capable of learning to be compassionate toward all sentient beings. And it's really important that we take that seriously.
Zen teaches us that it is all karmic. Our meditations allow us to examine our emotions and our thoughts (which are all creations of our past karma).
I'm not sure that anything is real. But I do think that one of the keys is to examine our emotions and reallywatch how that all works. Those destructive emotions of anger and greed and competitiveness and fear are the cause of most of our negative karma. Redirecting that energy into compassion makes for a much easier life.
Those strong emotions drive your thoughts so it's important to really watch your thoughts as they fly by. You can tell where you need to work just by listening to those thoughts.