Manga about teenagers making a suicide pact online I've never actually read the manga, only the fanfic of another manga it inspired. The author couldn't remember the name of the manga but here's what they had to say about it.

"It had a group of teenagers who planned a suicide pact online, became friends, got distracted by alcohol and general shenanigans, and ended up not going through with the suicide anymore... main female lead was out with one of the guys and she was thinking 'maybe in different circumstances, we would have been out on a date instead of out planning to die'."

I took out some stuff where the ellipsis are, since it was a side comment. They did mention the ending was kind of tragic though. I'd be really grateful if someone knows of it. Thanks in advance!
Sounds like the first story of Kanojo ga Tonda Hi.
This is the description of first story from MAL Page of Kanojo ga Tonda Hi:


  “Recruiting those who want to finish it and die, but feel sad doing it alone…”
  Fed up with everything, Mie posted a notice on an Internet Bulletin Board and found suicide-partners. Four boys and girls were gathered together because of their ties to death. And then…?!


The continuation from that description is just like the description you have:


  ...became friends, got distracted by alcohol and general shenanigans, and ended up not going through with the suicide anymore... main female lead was out with one of the guys...


I'm not really sure about her thoughts, but indeed she thought of something along the line "If only I met him earlier" or like that, and she did fall in love with him.
And about... 


  ...They did mention the ending was kind of tragic though


maybe it's about:


   Just like you have said, the suicide plan was nulled. But in the end, one day, the three of them got the news that the other girl (the one who's not the main chara) was dead, from committing suicide.