Two things to work on right up front. You are anticipating the shot with your bow hand and grabbing the bow. Watch your bow hand grasp at the bow when you shoot. Your bow hand needs to be dead. Let the bow react.
The second and most important thing you need to do, before you develop a very bad habit that you can't break, it to stop snap shooting. Control the bow. It doesn't matter how you aim but you need to control the bow. If you snap shoot too long you will develop a trigger and when you hit it you will let go of the string. I know you said your bow is too heavy, but you really need to pull with your back to anchor, sustain pulling at anchor, get on target, then increase pulling to conclusion. Once you are settled on target start contracting that back and simultaneously relax the fingers.
Right now you are pulling with your shoulder/arm. Watch the string arm elbow when you shoot. If the back is engaged you'll see the elbow go back and down at a 45 degree angle. There will be rotation in the back when it's engaged.