David,
First off,you don't have to resign yourself to anything (let alone being a mediocre shooter) you've proven this already by asking questions.
As Capt Eddies says,by your own description "your over bowed" ,but don't worry just yet.
From now on,every morning and every evening,"draw and hold your bow at full draw with an arrow on the string for as long as you can, then let down and do the same again.Put a time on it,like if your only really comfortable for one second now,make it 5 seconds,do this until you can't hold for more than one second,,,it will only take 3-4 full draws at first,but you will slowly build strength and within about 2-3 weeks you "will" be holding 30 seconds or more.
Now go and buy a good instructional DVD,,,,,from Ricky Welsh,"Shooting instruction II" to be exact.
Then stick to his advice and his advice alone,forget what ever else your doing "stick with what he says",stop the trial and error thing with your anchour point,just stick to the "one" method until it becomes second nature.
Now,one other thing,,,"Fred Bear had no faults"
Good luck and welcome aboard.