What you don't show is (IMO) more important than what you do. If you build up the scene right, you won't have to show any actual suffering going on at all. If you think of the scene in Reservoir Dogs, when the character polayed by Michael Madsen cuts off the cop's ear, we don't see the guy's ear getting cut off (at least, not in a clear-focus shot), but because the scene has been set so well, it's absolutely horrifying. I recently read a horror short called 'Proboscis' by Laird Barron, and he does much the same thing. The most horrifying things actually described are someone grinning (only becomes horrible later, when you work out why he's smiling), and a car with a darkened interior rocking back and forth. We get to see one pale hand pressed against the back window--but that's it. And it's a really creepy horrible story. I loved it.) Same applies with monsters (think original Alien).
Short answer: when it comes to suffering, things that people imagine for themselves are much more horrible and likely to elicit sympathy for your character than anything you can actually describe. IMO, I'd think about skipping the actual suffering itself, and focus on the build-up and aftermath--and use the aftermath for the character changing moment. Just my opinion, though, and it may not work in your story.