I could also cry reading your post Waggers. My daughter is similar to your son at times and is also a terrible sleeper. It's a special hell.
I agree with everybody that has said to get him into more daycare, you need to recharge and step back out of the chaos of it all, as others have said he will be safe and cared for, if he is awful in the evening, you and your partner could tag team each other. At least you would have been able to recharge while he was away, at the moment you are clearly in the eye of the storm.
I'm so heartened that posters that have been through a similar time have been quick to offer their understanding and support, you are not alone or a horrible person, the parenting gig is a hard slog, throw in no sleep for nearly two years and challenging behaviour from your lad, it's no wonder things have come to a head.
You are heading in the right direction going to the doctors today, honestly arrange more daycare and get yourself well, things will improve, heck if you have to, go back to work full time for the break.
You will come out the other side of this, you will!
I remember you from the sleep thread too and you are an older mum like me.
Keep posting, you know the ladies will have your back every step of the way xxxx