The words ‘weaknesses’ and ‘flaws’ both can apply to traits that can be improved and traits that cannot be. ‘Flaw’ is wrong here because English prepositions aren’t part of you. Her biggest flaw is her rudeness. (It’s a trait of hers) The most common flaw/mistake I make in English is using the wrong preposition. (‘flaw’ is a pretty strong word. ‘mistake’ is more natural) English prepositions are my weakness. (I’m bad at them) English prepositions are my bane. (They annoy me)

Hi Yuliya, I would say the use of flaw in this context is incorrect. Prepositions aren't part of your personality or character. Grammar of a language is something we acquire. Instead, you could use weakness /weak point. Hope this helped. Feel free to message me anytime with any other doubts!