I find myself in a similar position with regard to being overwhelmed and unable to get consistent control of the GTD implementation.
It really does help to allocate and stick to the review time, as it is certainly false economy to continuously respond to "firefighting" tasks and distractions. I think that sometimes it is easy to fall into the habit of dealing with the first thing at the front of your mind instead of emptying your head and using the review time to prioritise more effectively.
I have fallen victim to over thinking at times rather than cranking through my NAs. I don't always drill down far enough into more granular NAs, although this worry may be the cause of of procrastination, to some degree.
I have set myself a 30 day challenge to apply GTD consistently each day and get it up and running, once and for all!