Gee, Woodduck, you sure got lucky. Glad to hear you're back up and climbing after your serious accident. Thanks for posting about it here.
We agree with the lesson you learned, but we think it's just as important to inspect the tree thoroughly before you climb it! I see you wrote that it "looked sound," but clearly that wasn't enough. Have you read our article on tree inspection? (It's in the "All About Tree Climbing" section of this site under "The Tree You Climb"). I think you'll find it useful.
All the best,