I am really hating on myself today. I recently missed an assessment at Uni because I chickened out of doing a presentation (even though it was only to 2 of my tutors) and after having a meeting with my tutor and explaining my situation she gave me a second chance to do it and said that it doesn't need to be a formal presentation and that I shouldn't worry about it too much etc etc, so that was good and it meant I didn't have to fail the module so I was pleased that i had been given a second chance. But then what do I do? I miss that one too. I am now really angry and upset at myself and I don't know what to do. I feel absolutely useless in every way. Why am I like this? I avoid absolutely everything, all the time. I don't know what to do and I just feel like packing a bag and running away
I can absolutely relate!! I had the highest grade in a class years ago until it came time for me to do a presentation. I did the written work and pushed it under the professor's office door, but never went back to class (out of fear that the first day I went back I'd have to do the presentation) and ended up failing. Unfortunately, in the moment, it is really hard to handle pressures like this sometimes. Currently, I am avoiding studying for a class, reading for another, and doing a bibliography for a third, mostly out of lack of time, but I know I'll regret it if I don't get these done in time (at least there's no speaking involved).
I always hate myself after avoiding something that wouldn't be such a big deal to somebody without SA, but is practically impossible when you're consumed by anxiety and fear. However, after facing a situation like this, I feel a huge sense of accomplishment. If I were in your situation (I've been in similar situations numerous times), I'd want to avoid the feeling of regret. If it is possible to make up for what you've missed, do so. If not, find a way to move on (find out all options first--I had two teachers suggest to me to drop classes because I had missed so much, and I appreciated their honesty, but the same semester, had two willing to work with me to continue). The next time you encounter this, if you're able to, discuss beforehand any difficulty you may have (I often worry if I say something about my anxiety after missing something, it will just sound like an excuse). Maybe you can work out a plan to make it easier for yourself. I also like to reward myself for facing difficult situations. If you have something to look forward to afterward, maybe it willl feel less daunting. And, definitely recognize any progress you make. Baby steps can go a long way over time.

Best of luck!