Why am I upset? It's because every time my friend promises me we'll hang out,

We can't hang out because he's "tired" of covering at work for all those other people.
And literally the next day he went out with our coworkers. Including the bully who's a two-faced b.itch that we both agree we hate. Huh? And now he's put up a picture that my bully took of him. It disturbs me. It's as if I'm being mocked by everyone including him. I don't understand if this is my mental illness talking or if it's genuinely how it is.
Because there are times he's really a great friend and will put me before other people. But when he doesn't it speaks volumes. It's like he uses me as a pawn sometimes I guess. I don't understand him.