Most younger players (college on down) that develop a similar problem to yours is caused in their legs. Their legs get “lazy” The reason players “open” up is not because of their upper body, but because of their lower half.
Suggestion - Throw on a line. When throwing with a partner or even into a toss net, position yourself on the foul line or some other “drawn” line.
Start with a “three step” throw. Put the line between your feet and your feet should be a normal separation (athletic stance). Step with your glove side foot towards the target. Make sure you are “walking” down the line still centered under your body. Then as you take your step with your throwing side foot, rotate the foot to be perpendicular to the line | . As you take this step your shoulder and front side elbow (remember front side is direction, back side is power) should be pointing down the line to your target and your chest should be facing away from the target at 90 degrees. Now as you take the third step, make sure you are still centered over the line and place the glove side foot close to the line on the side you started from. (Left foot lands just slightly to the left side of the line)
This may seem “common sense” but many times when throws start getting “out of wack” we have to break it back down and “re-train” the body and the mind (which makes our body do what we want).
Start throwing from about 40 feet and move back slowly as you begin to hit a target the majority of the time (19 out of 20 throws), If you are throwing to a person, have them make an “O” with their arms with their hands above their head. THAT IS YOUR TARGET ZONE. That give you a target from about their heart to about 5 inches above their head and about 6-8 inches to either side of their ears.
Dont work on velocity… work on your lower body position and releasing the ball to accurately throw it to the target.
I do this drill with all my players. In fact, my son does this everyday. BTW… when we go to the Jersey Shore every year, he is usually restricted from the baseball throwing game after a couple nights as he breaks bottle after bottle. No joke…
Accuracy comes from your front side in throwing, but if you dont move properly to get your front side in the correct position, your body will compensate and “open up” in order to “make it” throw the ball in a direction you are trying to throw.
Without seeing you in a video for break down what you might be doing this is just a suggestion to break down your throwing to a proper starting place and build it back up from there.