What you are suffering from is a form of target panic. When you are at a distance where missing isn't an option you can control your shot. Once you get further and the element of hitting the target is introduced you no longer command the shot. Sound about right? If I had to guess you probably shoot quickly to start and after going to the 55 pounder it probably started getting worse.
The thing you'll have to do is reprogram your brain to divorce aiming from shooting form. Right now your the aiming portion of the shot is controlling your form, irregardless of whether you're in the position to accurately execute the shot. I would start off by dropping down in draw weight to something that is so easy you can control it with no effort. Start close to the target and work on establishing a shot routine. Once you feel comfortable with that begin by stepping back. Close your eyes, draw your bow, and settle into an anchor point. When you can do this, open your eyes, focus on the target and fire the shot. This will help train your body to recognize the shot mechanics separate from aiming.
After you can do the previous drill you can begin to work on shooting at the target with your eyes open. Focus on going through the steps of drawing and hitting anchor before you fully focus on aiming and releasing. If you feel out of control or panicky... stop. Focus on only making clean, controlled shots.
I had a friend who had the same problem. No anchor to speak of, premature releases, no control. I helped him with the same drills and it was a rough road but he did gain control over the shot. Good luck.