Upset about not being invited to occasions

Hey, this is an upsetting circumstance and while I don't have the most experience with this topic, I'm willing to give some advice.
Have you tried to schedule occasions with your flatmate and your friends? Perhaps they don't realise that you would be open to being invited, and by scheduling occasions with them, it might make them more likely to invite you back. You could try talking to them about this if you feel comfortable, but remember that people aren't mind readers. They might just not realise that you want to go to occasions with them.

Have you made any invitations to your friends and flatmates to go out socialising?

If you're friends berated you because you simply expressed yourself, then I'd question if they're really your friends.. If you're more recluse now, they might find it difficult to invite you places though, as they might think you're not open to going. I would try and schedule an occasion to socialise in, then see what happens.

I somehow feel my life has sunk into an abyss

It does feel like that when you're lonely, like there's nothing better to do than locking yourself in your room, as depressing as that is. It took me a long time at uni before I met a few friends, but it's falling apart a bit so I'm back to feeling down in my room

I have finished uni years ago. I am 27.

Oh so it's not halls then. Really not ideal to be living with people you don't get along with at that stage. Maybe find someone else to move in with or you'll continue feeling horrible