Q. Why couldn’t God just change things on his own? I mean, as powerful as he is to create the universe and mankind, living plants and mammals, I really don’t understand this part.
I understand you to be asking why God doesn’t act to end all the evil and suffering in the world, since He is omnipotent and no one can resist His power. I believe that this other post on my blog largely addresses your concerns:
“Why do some people seem to suffer more than others?”
Even though that post is written in response to a different question, it gets at the same issues you’re asking about. It explains that God created a world in which there was genuine moral freedom so that there could be the possibility of love. But at the same time, this freedom allowed for the possibility of destructive choices that would lead to suffering. Rather than act in all of His power to end that suffering (which would require taking away moral freedom), God chooses to work through the suffering to bring about His purposes in the end. He asks us to trust Him as he does this. And God Himself was willing to suffer, in the person of Jesus on the cross, setting an example for all of us to follow.
I hope this is helpful.