Here is what I am discovering at age 66 and recently retired. I was always a very good player, won a lot of tournaments in my 30s, 40s, 50s. I actually feel that I AM A BETTER PLAYER now, but like you, I have struggled with PERFORMING, where I know I can be much better if I played competitively at my true potential.
Having the time now to really analyze myself- I do believe that it is all mental. I have no problem with eyesight, stamina during reasonable hours of play, and I am in great shape physically, most think I am at least 10 years younger than my age.
So with that in mind, I have focused 90% of my practice on being POSITIVELY COMMMITED to each shot ; once I complete my pre shot routine, ( sighting the shot, deciding on speed, english if any, and stroke to be used) - I get down on the shot- a couple of warmups, find the target point on the object ball, and then let the cue go while staying focused on that object ball. Your internal self -CONFIDENCE, will allow your relaxed back hand to deliver the stroke as planned . All of this takes maybe 10-15 seconds. Then you repeat this process over and over again.
For me, this routine keeps me focused to the point that I am almost constantly in a dead stroke zone if I am following this process. when I find myself missing shots that i know I should make- I just immediately remind myself to go back to this process and it keeps me from getting way off track.
I find with age the ability to focus repeatedly for longer lengths of time is diminished - it makes sense- no way an older mind can function as well as a sharp younger mind- just like everything else about our body as we age. So I find that developing more FOCUSED pre- shot routines, as least for playing pool, results in keeping the mind more focused for longer lengths of time. I couple this with a VERY POSITIVE attitude when faced with any shot- I find it imperative to treat EVERY shot the same, no matter the difficulty- it helps with confidence and focus. I just don't stress over anything at the table anymore- it is all about trust, focus and execution, while keeping mind and body in a relaxed state.
We are all going to miss shots, but when you do, even if you thought it should have been made; very important to just remind yourself to get back into the routine for your next opportunity at the table.
I will promise this, if you can get this process down, you will be running racks of rotation games and 14.1 and many times not even be aware of how many racks you are running - I remember 30 years ago taking lessons from a young Mike Zuglan (JOSS Tour Guy), in 1992 he was one of the best under 40 14.1 players in the U.S. - he told me that when playing alone he never knew how many balls he ran prior to a miss- he did not count racks, he was capable of being in that zone almost constantly!