Complex sentence: Although his nerve failed him at the tryouts, he accidentally stumbled into the intimidating challenge that determined eligibility and suddenly found the confidence to take the challenge, with his skinny body proving to be an asset that enabled him to succeed.
Split into simpler sentences: Although his nerve failed him at the try-outs, he accidentally found himself in the intimidating challenge that determined eligibility. Suddenly, he found the confidence to take the challenge and his skinny body proved to be an asset that enabled him to succeed.