This happens! It's a common element of the experience of studying Verbal as a native English speaker. Think about it like this:
When you first start learning the rules, you'll be clumsier and slower with the rules than you were with your ear. That's to be expected - you've been using your ear for grammar for your whole life, and you probably haven't been thinking that much about grammar rules.
So, why bother learning the rules? Because it's very hard to improve your ear (the only way to do it, really, is to read
a lot), while it's much easier to improve your rule knowledge. So if you use grammar rules more, you're putting yourself on a path where you can improve more quickly and more easily, and improve to a higher level.
That said, in your case, I have to ask why you're putting any real time into studying. If you were able to get a 760 on your first attempt, why not just go take the test now? 760 is a 99th percentile score; there's no school out there that'll treat a 760 differently from a 770 or a 780. It's more than enough for even the very strongest programs.