You're writing along, feeling fine, everything's great and then suddenly BAM. There it is. The wall of suck.
For me, I tend to hit this wall at around the 2/3 mark when I'm still far enough away from the beginning and the ending that I feel lost and like I don't know what I'm doing. It feels like I've been wandering through a desert with no problems for weeks then suddenly, I remember: I AM IN A DESERT AND GONNA DIE. Everything runs into a panic and I sit in paralyzed anxiety, too afraid to move forward or backwards.
I'm a linear writer so I tend to follow my story along from beginning to end and once I'm muddling through the middle, I start slowing down and then hit the wall. Sometimes it feels like it's three miles high and made of glass shards that cut if I get too close, but it's clear enough that I can see through. I know what's on the other side but I'm afraid of breaking through to get to it. That's the wall of suck.
What do you do when you hit it?
The answer's easy, you push through that sharp, nasty bastard and keep going. Army crawl under it, fling yourself into it, or climb over it, but you've got to keep moving. If you stare too long, the wall only gets worse because your mind makes it worse. That wall is your fear of sucking, and the truth of the matter is your first draft probably does suck. It probably sucks a lot. And that's okay, and normal. No one (okay so a few magical writers) writes a perfect first draft and it's okay to not be perfect. Remember, the goal is to finish, not to write the most beautiful and grammatically perfect sentence the world has ever seen.
The goal is to drag your battered, bruised, and bloody self across the finish line and scream that you did it. So, put on your helmet, buckle down and show that wall that nothing is going to stop you.