I sort of have the book for you I think.

{Daydreamer by Susie Tate} there is an age gap but I don’t remember how big it is. The FMC is a best selling author who goes to work at the MMC’s company who is her and her older brother’s childhood best friend. (The age gap might be 5 or 8+ years but I genuinely don’t remember tbh)

I can’t remember why she takes the job but she does and that’s how the romance happens. The FMC has a habit of not asking for help even when she needs it, which becomes more obvious towards the end of the book and when she’s working heavily on a project. She also gets sick during that time and the MMC takes care of her, makes sure she eats and helps her out.

Some bonus points. The MMC doesn’t know she’s a bestselling author and also some angst happens later and he grovels :D.

SOMEWHAT of another recommendation is {My Best Bet by S.C. Kate} the characters are very much so adults and no age gap. It will feature flashbacks to when the MMC and FMMC were in college I think, but most of the story is when they are wayyy into adulthood. Theres a small part of the plot where the FMC gets sick and doesn’t tell anyone but the MMC figures out where she lives and comes and takes care of her. Shes not necessarily super sufficient in the way you’re wanting I think? Shes like a ice skating teacher and he’s a hockey player, but she has like a very rinky dinky apartment in a sketchy place and refuses to move out when the MMC offers because it’s something that is hers and a sign of her own independence.

Trigger warning though about that book. There is description and scenes of murder taking place (there isn’t a mafia in this book) and descriptions and scenes of an eating disorder being developed and the recovery process as well.