[color=red]'18+'[/color].

I'm looking for a yuri manga about a highschool (?) student who falls in love with an older woman (named Hitomi, I think?) who she often sees on the train. One day the student just decides to go in for a kiss, Hitomi (?) reciprocates and they start dating. It's mostly just smut iirc. There's a public intimacy scene where they're both sitting on barstools, a hotel chapter, and some pee stuff in a park in the last chapter.

Release date: 2013 or earlier

Length: around 5 chapters, no more than 1 volume (also not sure if it's ever been published as tankoubon, it didn't have a colored cover)

Scanlation group: possibly Lilicious?

I Fell in Love For the First Time (Hajimete Koi o Shita. by Asagi Ryuu)

I'm surprised you couldn't find it with how much information you provided.