To me it seems like the best thing for you is to hang out with other people from the group,cause you deserve to do what's best for you and avoid all this anxiety.I hope there is at least one person that will understand why you can't be around her and would gladly hang out with you,if not all of them.Personally I would either keep being their friend like nothing happened but without trusting/expecting anything from her or I would be like you are right now,cause I know how it is when you can't be around someone...

literally going from doing everything together to pretending i don’t exist while we sit in the same table. her actions towards me made me feel invalid, insecure, insignificant and my mental health took a big dip during a very crucial exam period.

i felt completely wronged and didn’t think that she was worth having a friendship with at all. this is likely a selfish and immature move but i wanted to put myself first for once.

It's not immature.Pretending that someone doesn't exist is immature and not giving an explanatiom for months.It is completely reasonable to feel the way you do.I know that doesn't help but maybe you need to hear it...Do whatever you want.You feel like staying home? Stay home.You want to avoid her? Avoid her.If one day you feel like it will be interesting to be with all of them,do it.