I went to go see the movie Secret Window when it first came out in the theater in 2004. I enjoyed the movie on a lot of levels, but particularly a scene early on wherein Johnny Depp's character, a writer struggling with his next novel (among other issues), is working at his computer. "This is just bad writing," he says in frustration, looking at the words on the screen. "Just bad writing." Highlighting everything he's just written, he hits the delete key. "No bad writing."
You have to be willing to do that, to just chuck out all the work you've already done, if the little honest voice inside you tells you it's crap. (Not to be confused with the little self-conscious voice that tells you it's crap. You can't trust that one.) Sometimes you read through a section and you know, deep down, that it's not good. You don't want to have to rewrite it. If you're like me, rewriting from scratch is, while not impossible, incredibly uncomfortable. I have a difficult time re-envisioning a scene, and each time I rewrite it's harder for me to get into the character's head.
Just because it's annoying and difficult doesn't mean it's not worth doing. Suck it up and start over. You may need to give yourself some time away from the project first, or you may need to break it down into manageable chunks, but do it. Your manuscript will almost always be better for it.
And if it's not; highlight, delete, and try again.