Not having a suffient taper - too many races too close together?
I had an absolute pants race today - I know I'm just getting over a cold and ran a hm two weeks ago when I had the cold and had a poor run then, though better than today by a long shot.
When I finished I just wanted to sleep, I also know that my rhr is elevated still, don't run with hrm but I know that my hr is higher than normal as I can feel it. All I wanted to do post race is sleep - for 13 miles thats abnormal.
Don't knock yourself over one bad run, though I know yu want to because I am right now.
It really does sound like you are eating / drinking too much before the race. I have max one bottle of water before a hm. Try not to drink during a race, just a mouthful of water and spit it out.
What was the terrain / elevation like? Did you hit the hills too hard? Just bog down? So much can go wrong.
Back to the drawing board.