I agree with a lot of this piece by Oliver Burkeman on the overemphasis some place on Positive Thinking. I don't think Negative Thinking as the default state is all that much better. Some situations warrant Positive Thinking, some warrant Negative and some warrant simple logic instead of a blanket application of optimism or pessimism.
If Anthony Robbins pointed to a bed of red-hot coals and told me to walk across them with the right attitude and I wouldn't get burned, I think my response would be along the lines of "Don't be ridiculous." I'd figure that while the trick might work, it might not…and there was no possible "up-side" to its success that warranted risking the "down-side" of its failure. It's fine to take risks that make sense but some simply don't.
I saw an interview of one of the folks injured on that recent Robbins firewalk — a lady who said she was trying to learn to go through life saying, "I will succeed. I will succeed. Failure is not an option." If that works for you, mazel tov. What seems to work best for me is to have a pragmatic assessment of any situation, to recognize that failure is an option and that it's up to me to try and avoid that option. Positive Thinking just makes me forget that it's up to me to actually make things go right.