You spend hours/days/weeks editing and you're struggling to get through it. Here is just one of the many tricks that I employ when editing both my own and clients' writing.
Use Paper or an eBook Reader
I never do all my editing directly on the computer. The back-lit screen is too much on my eyes. The smooth reflective surface will catch the overhead lighting, adding to the eye strain. This is why I don't use a tablet for editing either. They too are back-lit and reflective. No, when I can, when the size of a document permits me, I will print out the manuscript and pull out my trusty red pen, writing all over that printed document. In those instances where the manuscript is too large, say 100k words, then out comes my Kindle and I'll read the document on that, adding comments and notes to the file as I go. (Yes, you can do that on a Kindle.)