Hello Monkeycar, and thank you for opening up to us, I know that it's never easy and it's something in which we tend to look at our computer, at this particular site and toy with the idea of posting a comment, perhaps type something out, then edit it and maybe delete it, especially when we aren't feeling the best only being annoyed at ourselves in doing so, so I am very sorry along with Sophie.
Hiding your thoughts while you are at work is difficult, pretending to laugh at what another person has said, where you find no humour is a pain, and not only that you can only work at half your usual compacity, unable to concentrate on what you found to be easy is now most difficult.
There are other avenues that someone believing they are the black sheep can adventure in, it's just knowing that you're able to do this, but you need help to unlock this padlock, you can't start by yourself, only because you just go around and around in circles, digging yourself deeper and making your situation worse.
Don't get me wrong here, I've been in the same position and know how incredible this feeling is, that only you know how it feels.
I know you love your cat with all your heart, but can I make a suggestion, and that's to get another kitten, today or tomorrow, you need to have another cat which you will learn to love, this doesn't take away your love for your other cat and could be a reason why you might smile and that's a start.