[toomuchtodo]

> but because it's less painful than being rejected by the cliques.

Have you asked to sit with these groups of people and been verbally rejected? To belong, you will need to make the social effort.

EDIT: Hey folks, I'm not trying to be rude. I am genuinely trying to help OP. If you want to join a group for lunch, walk up to them with your lunch and say "Would you mind if I join you?" Don't wait for an invitation that might never present itself. You have nothing to lose except a possible rejection (which is important to learn is minor).