If you liked the Natsu e no Tobira manga, the Anime-Planet community thinks you'd like:
Both these manga are short, female-oriented stories about May-December romances. They don't have many similarities besides that, but the subject is fairly rare in manga, so check out both if you're interested in cougars and their much younger suitors.
Gorgeous 70s-era shoujo dripping with melodrama, forbidden love, and beautiful tragedy. If you're into this genre (and if you're reading this you probably are) then I highly recommend reading both of these.