Hi there, I'm also autistic so feel as if I could give you some advice? Although, all autistic people are super different so apologies if this is no help at all!
The eating thing is definitely the biggest issue here. Going into the canteen alone would certainly be stressful, but is it because of the people there (the social aspect) or is it because of the noise and crowds (the sensory aspect)? I would recommend just putting headphones on, going down, ignoring absolutely everyone, and getting food quickly.
There is time to work on these social issues you're having, but not eating for a week is not something to make a habit out of.
As for the social aspect, it is difficult talking to people, even saying hi can of course be stressful. I know that I still have no idea how to start a conversation with someone unless we are in a situation that I can discuss with them. Even just giving people a wee smile in the corridors will slowly boost your confidence and make you seem like a more approachable person. A smile and maybe a small wave can help make people see that you don't mean to be unapproachable and that you are a nice person. Working on social skills takes time, and rebuilding the ones you had is a great place to start. You won't be doing anything you haven't done before, which should make things easier. Again, I recommend working in very small steps and slowly building that confidence back up
Reading the part about having no identity and personality really made me sad, as I have felt the exact same way many a time. It is something a lot of autistic and neurodiverse people struggle with. All I can really say is that spending time alone has really helped me with this. During lockdown I was basically alone the whole time, and so had no reason to mask. This made me realise that the front I put on with people is actually not at all who I am. I have gotten to know myself so well and no longer struggle with the idea that I have "no personality". I would recommend spending some serious time alone, figure out your likes and dislikes, your opinions, your views, without any influence from others. You will really get to know yourself and eventually realise that you absolutely have a personality, it has just been hidden for a while due to masking!
I hope this could be of some help to you, although it may not be depending on your circumstances etc. Maybe try talking to a professional if you feel it would be beneficial? Although that is easier said than done. I hope you feel better soon!