What is the difference between してしまいそう and しそうだ


I've come across this sentence in Shinkanzen Master N3 Reading, and I don't
understand what the difference is. The sentence goes as follows: 遅刻してしまいそうだ。it
then asks what the difference would be, if the sentence were 遅刻しそうだ。 I would
greatly appreciate your help. Thank you.

