https://www.mangago.me/read-manga/bait/ (I don’t see this as a bad ending tho, it’s perfect for me)
https://www.mangago.me/read-manga/on_the_lake_where_the_waves_reach/ (This one tho
Prepare yourself with them
https://www.mangago.me/read-manga/haruka_tooki_ie/
basically two boy with asshole dad in love but the people around them don't approve of their relationship
https://www.mangago.me/read-manga/10_years_where_i_loved_you_the_most/
After 10 years in a relationship, the uke found out the seme is cheating on him, there's more but I don't wanna spoil you too much
Ambiguous/Open endings are fine too.
By bad endings I mean either break-ups, death, amnesia, whatever that's awful.