I think a mistake a lot of ppl make when writing ship fanfics is that they erase the character's actual mindset, goals, or ideas in the process of making the story fit their idea of the ship more. When in reality the characters relations with the other characters around them, especially ones that they're close with, and their ideas and veiws are super important to keep the ship and characters believable and to make a good romance fanfic. But I'm not hating because it is really hard to do when writing about someone else's character and not ur own, I make this mistake all the time, so I've tried to analyse the characters a little bit more to make it better and stuff but it'll never be prefect because it's just not my character so I can't write them perfectly :]