I'm finding it difficult to write about anger and pain, sadly, two things I know a lot about. The anger mostly comes from an inflamed sense of injustice and the impotent desire to change it. Pain, well, 30 years of headaches and migraines is enough of an explanation for that.
The problem with these two concepts is simple: readers often can't identify with them on the personal level that I do. My first novel ripped into these with a vengeance. The novel is long-winded (200k words) and filled with long stream-of-consciousness sections where the MC tries to make sense of his feelings. Now, that novel will never see print. It's part auto-biographical and part supernatural horror. But I wrote it for the need to express those things. I'm finding anger a much easier emotion to write on now that I've "gotten it out of my system," but it is still a very difficult emotion to portray in a way that doesn't push the reader completely out of the story. Going on an "F-bomb" rant in the privacy of my own bedroom can help. Doing it in a book creates a hostility the reader might simply draw away.
As for pain, it is also a difficult thing to transcribe into written form.
The helpless feeling of a pulsing wave of sickening agony as it rips through your head, each heartbeat more devastating than the last, until you start to wonder if it might be better to simply skip the next heartbeat, and the next, and all the rest that come after.
See how that sounds? Whiney, pathetic, craven. Is that a character you'd want to cheer for? But I'm telling you, that's exactly how it feels some days. Getting that into a story without creating a pathetic character is extremely difficult. My MC in my current project is going through increasingly bad headaches and its been a challenge to keep it toned down, especially since it is a first-person narrative.
So, these two emotional moods are my biggest challenges and, hopefully, my biggest rewards since they're teaching me better ways to express ideas.