Not doubting your ability, we've all been there from time to time.
You said that you checked the length of the old belt to the new one. So that's not it.
Was the old belt broken? If not, how did you get off? Reverse the procedure.
Having done a few drivebelts in my day, the frustrating one's are usually due to being Very difficult to get your hands and or tools in there.
But then there's the odd one. It basically comes down to the fact that you're not seeing something. Sometimes it's obvious. Sometimes not. There's an adjustment somewhere that you're probably not seeing. Been there, done that. I've pulled off A/C units and power steering pumps to try and fit a new belt... only to find a hidden adjustment bolt.
It sounds like in this case, in your case, you can't even get it around the 2 main pulleys, with or without the tensioner.
Good luck.
Edit: For what it's worth, the second picture in your first post. That belt is basically on there, by that picture. You might have to manhandle it a bit, but don't use any sharp tools.
Or determine the correct direction and bump the starter once you get it to that point. Better yet, do like
@85Hokie suggested.
One other thing to try. Heat up the belt.