Do not see this as a failure. It is not. Companies do this often and when perceived realistically, they have the right to expect solid performance from their employees.
I was fired twice, without any severance, because I was not able to make it to work after my dad passed away due to my depression. I wasted a lot of time being angry at myself for not being more resilient, which was a mistake.
I was being too hard on my self. please don’t do what I did.
It seems like they are trying to avoid a lawsuit; otherwise they would not have paid you a generous severance. This is actually irrelevant to your pain, so I am sorry for going off topic. I am glad you have the financial safety otherwise your anxiety would have tripled immediately.
But, please don’t waste your time and energy being angry at yourself or the former employer.
I wish you a peaceful healing process and an empowering recovery.