Suffering is extremely difficult. There’s a lot of emotion involved, and that’s 100% okay. I’ve found myself mad at God before, and I’m sure you can relate.
There are some Christians (including pastors) who say it’s a sin to be mad at God. When I hear this, I always want to ask: Have you ever read the Psalms? The Israelites obviously had no problem expressing themselves to God. The psalmists laid themselves bare, so to speak, before God. They didn’t have a filter. And they didn’t expect God to be angry with them. It was just a natural expression of their faith.
So no, I don’t think it’s necessarily a sin to be mad at God. Maybe it reaches sin if you’re just blatantly cursing out of a hateful heart (although I’m still convinced God would rather people yell honestly at him as opposed to being afraid to say anything to him). But a child of God laying his or her heart bare before the Lord? I think God loves that. Jesus said anyone who comes to him he will never cast out.
I pray that laying it all out with complete honesty, just like the psalms, would become commonplace and natural to our faith.