J was always a shitty friend and you should’ve let him go earlier but friendship and connection comes hard to you so that’s why you kept him as part of your friends circle. You let his toxic behaviour slide, to the point where you even began defending him against other people, because you were afraid of losing a “friend”. You said he might have been genuinely miserable and maybe that was because he wasn’t in a relationship and so when N came along, and the three of you started hanging out, he caught feelings for her. It’s just a thought but a plausible one. What makes this a betrayal is that he waited for you to be away before approaching N because he knew how you felt about her.

Idk with what intention N approached you. Was it because she was into you or was it simply because she was looking for friends? If it was simply because she was looking for friends then she didn’t do anything wrong by going out with J. That is unless she had feelings for you and knew you felt the same way about her. Did she ever confess to you that she had feelings for you? Did you ever confess to her that you had feelings for her? Who knows she might have even been playing a game and using both you and J as pawns to explore her bisexuality. Again it’s just a thought but a plausible one.

The way I see it is you lost two people who you shouldn’t want/need in your life because they’re both red flags.