Mixing and mastering are two different processes/mind sets. The way you're going about it is like trying to put icing on a cake while it's still cooking. I can't think of to many commercially successful albums or singles where they were mastered straight off the mix buss. Maybe it's better for one offs or ref copies. It can be more of a work flow thing..to get away from the mix for a day and then approach it from a different view.