I'm not usually against age gap but in this case FL was a teenager, a vulnerable teenage girl with no one to relay on and not a lot of experience with life in general. If the romance started from her being 21 I wouldn't be bothered... And the fixation ML had on her since a little small kid was omit too. It could have worked since ML wasn't that terrible of a love interest. He realizes he's mistake and grow as a character should do.
Oh FL wanted to heal him with the power of her p**sy and being a submissive traditional house wife worked guess there's no need for therapy after going through a traumatic experience
The Reluctant Fiancee