Oh Sweetheart no need to apologise! It sounds like you've been having a really rough time. I hope getting it off your chest and into this forum has helped.
You probably don't feel much like reading at the moment, but when you do I'd recommend Russ Harris's book 'The Reality Slap'. It's all about dealing with unpleasant emotions and I found it really useful when I was going through treatment.
I lost my beautiful Tabby just before I was diagnosed. He was 16 too. The loss of a pet is a very special kind of grief and non-cat lovers just can't seem to understand our distress. It helped me to remember that my grief was a reflection of how much I had loved him. If I had never loved him I would not have been distressed, but if I could go back in time I would not have missed that 16 years with him. I'm sharing this thought in the hope that it might help you too.