Even though I've written some VERY long stuff, I don't really get sick of writing. However, sometimes I run dry and feel fresh out of ideas. If rereading doesn't help, I put it aside for a while.
You could try and make a summary (and possibly do a synopsis of the summary again to make it tighter), if you don't want to put it aside. That would work for knowing what's missing and such-like.
If you feel you don't see grammar mistakes or typos anymore (which happens all the time, of course, because we as writers already KNOW what we've written, so we can't help skipping a lot), then reading the thing backwards is helpful. Either word by word (last word, one but last, etc.) for spelling or sentence by sentence (last sentence, the one before, then the one before that) for grammar concerns.
no subject
You could try and make a summary (and possibly do a synopsis of the summary again to make it tighter), if you don't want to put it aside. That would work for knowing what's missing and such-like.
If you feel you don't see grammar mistakes or typos anymore (which happens all the time, of course, because we as writers already KNOW what we've written, so we can't help skipping a lot), then reading the thing backwards is helpful. Either word by word (last word, one but last, etc.) for spelling or sentence by sentence (last sentence, the one before, then the one before that) for grammar concerns.
Not sure if this is the advice you wanted?