Wow, this sounds a bit like torture for you. But you know... we all know... that sometimes what we build up in our head is 100 times worse than reality. It might be better than you think?
Maybe you can cut yourself some slack on this one? For example, only run the 1/2 and call it quits. I know sometimes it's hard to let go of goals that we set for ourselves... but unless your head is in this... starting very soon... it's going to be a hard, miserable race out there.
Honestly, you have proven that you can do it. You know you can do it. So, maybe this one can be one last horay for fun? Wear some mouse ears... cheer others on who are running... find a buddy to walk/chat with some of the way. Just go out there and do what you heart wants to do... and let it go.
I will certainly think of you and wish you luck. And when you are out there, think about this:
From Zia_Cyclist: Another aspect of mental training is recognizing the difference between pain and suffering.
Pain is an acute, hurting feeling that if you don't stop immediately will lead to injury.
Suffering on the other hand is that feeling you get when pushing your body to the limit. Don't get me wrong, it hurts. But, suffering isn't going to lead to injury.
When I read about this and learned the difference it lead me to push harder on some of my long runs and in races.
If it's pain... STOP! If it's suffering... push past it... or if you don't care to suffer anymore... STOP. It will be OK if you stop.
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather, to skid in broadside thoroughly used-up, totally worn out, and loudly proclaiming: WOW WHAT A RIDE!!!!"