The main guy wants a girl, girl rejects and goes out with other guy.
Later realises that first guy was better.
Tries to get back with main guy but too late, guy is living happily with other girl.
Girl Regrets choice
Guy + other girl live happily together.

Sometimes maybe like Kimi No Iru Machi except he doesn't go back to the main girl.

I remember reading this short manga with that, but I forget the name. I remember it not being very good.

Basically the girl befriends these 2 guys. A tall guy and a short guy who she becomes close to especially. Well she chooses the tall guy even though she's aware of the feelings the short guy has for her. Eventually she realizes that she loves the short guy, and breaks it off with the tall guy. By that point though, the short guy is trying to move on and has met this other girl and is dating her. The girl decides she doesn't want to break that up, but it seems like it hints at that the short guy still has feelings for the main girl if I remember correctly.

Do you mean Two Beasts manga? ~

¤ I'm a female human that want to explore the universe so I can find another Earth. ¤
♢ I'm a female human that want to sleep underwater cuz I find it surprisingly calm ♢
% I'm a female human that want to become a scientist but I can't because I don't like math, add-math and physics. %
@ I'm also a female human that just to stay in my room and wasting my time with favourite anime and manga. @

I'm also a female human that obsess with reverse harem related.