Ok, I'm going to suggest that your abbreviated arm action is caused by not having the timing to do anything else. And I think there are two causes for the timing issue.
First, while you do get started forward early, you also appear to lead with your front shoulder. That will likely lead to early shoulder rotation which will steal timing from the throwing arm. It will also limit the efficiency of energy transfer up through your body and rob velocity. Try to lead with the front hip and keep the head and shoulders slightly behind the front hip into foot plant.
Second, your glove arm never gets fully extended out front and it drops too soon. This also contributes to early shoulder rotation. Try to get your glov arm to mirror your throwing arm at front foot plant. This will give the shoulders the timing they need to stay closed and the arm the timing it needs to take a more full path.
One more thing... Tilting your head back probably creates a posture change that can also be contributing to your issues. But I can't say for sure with just a side view video.