How can I forgive a God I no longer believe in? I have a lot of anger toward this non-existent deity.
It seems to me that you do, in fact, believe in God and this is hidden from you right now. Why do I say this? You cannot have anger toward a person who does not exist. How can a person who does not exist be unfair to you and therefore hurt you? It is similar with God. How can you have “a lot of anger” for a deity when you claim the deity does not exist? Your emotions suggest to me that you do see God as real. If this is true, then you need to ask questions such as this: Is God perfect, all holy? If so, then God cannot be unjust to you. Perhaps it is people who have hurt you and you are passing this now to God (“God should have prevented this,” as one example). If this is your mode of thinking, then I recommend a deeper dive into theology so that you can address the issue of why God allows suffering in this world; why God allows others to be unfair to you. In other words, it may be the rigors of this world and hurtful people at whom you are angry.
Can I get angry with God for the mess that too many people make in this world? It kind of feels natural to me to get angry with God for all of this injustice in the world. It seems then that I can work on forgiving God. Do you agree or disagree with this?
Even though it may seem natural to you, your getting angry with God (over injustices which you experience from people) is not good theology. If God is all holy and sinless, then your forgiving God implies wrongdoing. I prefer keeping a sound theology and understanding that God allows for the free-will actions of people, even if those actions are unjust. People are the ones who behave badly, not God. Rather than forgiving God, I suggest that you try to practice acceptance of what is allowed and then to forgive persons. In this way, you do not diminish the attributes of God.