At Mount Horeb, what did Elijah complain to the LORD about himself?

Answer: He was the only one left

Elijah said Israel had torn down God's altars and killed His prophets: "I am the only one left, and they are seeking my life as well."

““I have been very zealous for the LORD, the God of Hosts,” he replied, “but the Israelites have forsaken Your covenant, torn down Your altars, and killed Your prophets with the sword. I am the only one left, and they are seeking my life as well.””— 1 Kings 19:10 (BSB)