From observing your form I deduced that;
1 - You need not grip your bow tightly, you'll torch the bow with a tight grip and the bow won't react correctly when the string is released
2 - Instead of pointing the bow to aim, try using the elbow of your pulling arm to aim the arrow. Sounds funny, but try it. Seems to work for me when I shoot. It will help keep your elbow down too.
3 - Try leaning forward a bit, as you cant the bow.
4 - A deep hook on the string helps prevent you from twisting the string which causes your arrow to fall off the shelf. Think of you hand as a hook, instead of puling the string with your fingers pull it with the hook that your hand forms before applying tension to the string.
5 - Not related to form, but you may try placing a small strip of leather across your shelf. Say an 1/8 of an inch thick and just enough to get the arrow off the actual shelf. This slice of leather used to raise your arrow off the shelf should be place on the shelf just above the deepest part of your handle. This will be more closely resemble your natural aiming or pointing of your bow hand.
This is just my opinion and some may scoff at my advice. I don't usually give shooting advice, but I felt that you have the desire to stick with traditional bows and I want to do my part to help you and build your shooting confidence.
As far as anchor points, I use one of my teeth. (second one back from my eye tooth) It is amazing that when I do not touch that tooth with my middle finger of my drwing hand, I do not group well or hit where I want to hit.
Most importantly, if you do not match your arrow spine to your bow, you will never hit what you want to hit. Just because your bow says 55#, it does not mean that is the spine of arrow you need. It depends on your head weight, legth of your arrow, etc. Sorry, you probably already know all of this, but I felt compeled for some reason. I think was inspired to do something when you mentioned that the nice weapon in your hand was not a hunting bow.
One other thing before I get off my soap box. Try shooting three fingers under. Some of my bows shoot better three under rather than split fingers. I shoot each bow hawever I find it shoots best for me!