"help it" - 'help' is used strangely, almost opposite to its normal meaning, in this very useful idiom.
"I can't help it" means "I can't prevent [myself from doing] it."
A: "Why did you drive into the river?"
B: "I couldn't help it. Someone removed the warning sign!"
A: Stop crying! It's not that bad.
B: I can't help it! (I can't stop crying)