Practice is the answer but remember good practice and bad practice achieve the same thing, habit.
Starting out I read a lot and talked to everyone I could about form. What worked best for me is having a friend watching me shoot from behind and checking arrow flight and form.
You mentioned anchoring in the corner of your mouth, this is a good anchor point, but take it one step further anchor at your eye tooth, it is more consistent and will not change. Little things like this will make a big difference in performance.
You said you are shooting nock high with the bare shaft, this is an indication your nock point is low. With a bow square what is your nock set at. I shoot a Silvertip and it likes 7/16” high typical for this bow. Take baby steps when changing anything with your setup, write everything down and make notes of what works and does not work.
I said practice is the answer in the beginning, step back at ten yards and practice hitting your eyetooth every time do not worry about accuracy. I still practice this once a week after 7 years of shooting traditional. Remember when it comes to form you are replacing gadgets like sights and releases. You have to replace these things with consistency. Place your anchor at the same spot every time and a good smooth release, Terry Green has the best example of a good release here on the TradGang site that I can suggest. Go to the shooters forum and you will get a lot of suggestions there.