Oh man. That's a horrible feeling- maybe the worst feeling in all of big-game hunting. So sorry to hear about it.
I had something somewhat similar happen many years ago. It had snowed, and I came upon a spot where two elk had bedded, a cow and a smaller animal. The smaller animal was bleeding. I surmised it was a spike someone had wounded. The way I found their bed was that they jumped up; so I started tracking them in the snow. The spike was bleeding quite a bit. I tracked/followed them for several miles, luckily they were headed more or less back toward my Jeep (I found the bed about 5-6 miles into a roadless Wilderness) because the weather was spooky and I don't think I'd have followed them AWAY from the rig. It was COLD, had got to minus 12 F the night before down at the bottom of the mountain and I was up near the top. Anyway my theory was they'd bed down again, and he'd stiffen up, so they'd let me get close enough for a shot.
They dropped down through a drainage and started up the other side, which was a north facing slope and had much denser vegetation... I was carrying a big bore lever gun and had the scope in QR mounts. I decided when it happened, it was gonna happen fast, so like a dork I took the scope off. I'm pretty decent with irons... or thought I was... sure enough they jumped up and sure enough it was a spike and momma and sure enough, I had a shot at about 75 yards. He was standing there looking at me over a log. Clean shot at his head. Shot right over him; I didn't put the bead in the buckhorn in the stress of the moment. Now he's moving (stiffly) through brush. I took two more shots through windows in the brush.
I must've hit him in the guts because now, added to the blood from his wound, was lots of watery diarrhea. Faaawk. Followed him just a few hundred yards when it started to SNOW and I mean snow. Ended up snowing about a foot that night and basically closed off the top of the mountain.
Not as excruciating as yours because at least I wasn't the one who wounded him FIRST and I guess for that matter I can't say conclusively I hit him in the guts....... but I'm pretty sure I did.
I've had a couple long tracking jobs on deer, but always found the deer... but it is NOT fun when you know you hit them solid but you can't find them.
Anyway buddy it's the SHÏTS when it happens and you have my empathy and sympathy.
So sorry to hear about it. 
