Proofreading errors are different from punctuation or spelling or usage problems, and you fix them differently. Punctuation, spelling, and usage are knowledge problems, and you fix them by learning. Proofreading problems are usually a matter of seeing, and you fix them by learning to look.

The better you read, the worse you'll proofread, unless you are consciously aware of what you are doing. Good readers and fast readers guess what the words are as they read the text, and they just check in now and again to see if they are right. The more they can guess, the less they have to look and the faster and more efficiently they read.

To be a good proofreader, you have to go back to being a child again, or pretend that you are just learning to read in English. Look at every word as it comes along.