The third friend

I'm sorry you feel this way but i know exactly what you're talking about.. I'm going through something like this too. It feels like I'm the third wheeler and don't belong with my friends which makes it harder to talk and enjoy myself when I'm with them. I keep thinking about how they both are closer to each other more than I'll ever be. Hanging out with them starts feeling like an exhausting chore

i feel like that all the time ive always been excluded and and left out in groups.

idk ur exact situation but try to interact more with the people in the group. i usually send them a casual text and start a conversation, that builds ur relationship with those people. if youve been talking to them for a while and u notice they never text first, seem disinterested in the convo, dont put effort into ur relationship then its worth talking to them.

something i do to avoid being left out and being the third wheel is just make friends with one person (i dont mean only have one friend tho) have a friendships where its just u and them.

its easier to do this with the quieter kids. and usually the quiet kids needs the attention and lots of them can be pretty nice ppl when u get to know them.