All these posts and nobody has touched on what I perceive to be your biggest problem which is that you are losing all control of your glove side and flying open with your front shoulder. IMO the pictures you posted look entirely different from the video. However the one thing they do have in common is that your glove is dropping below your waist in all of them. It really looks like you are pulling with your glove side elbow when really, it should just close to your side. For example, in the picture of you wearing the yellow jersey, your body position looks pretty good IMO. The problem is what that glove side is doing. If this were video I think we would see the elbow go straight back pulling everything off line. Instead that elbow should be closing either straight to your side or out in front. Doing this should help you maintain postural integrity (which is really good in that picture) and you won’t fall off to the side.
Here’s a couple points regarding the previous critiques on your mechanics:
I think there is more external rotation there than meets the eye. It’s deceiving because your high arm slot has caused your shoulders to tilt and that shoulder tilt makes it difficult to see your arm angle to evaluate external rotation. I am not disagreeing with JPeavy, but guaging external rotation with a high arm slot is difficult from the 1B perspective. For instance, if you look at the head on shot that you posted all of the angles look vastly different from your 1b video
Stretch your hand to the catcher/Get Horizontal/Use your legs:
I think everything that Pitcherdadfan thinks he sees can be fixed by staying closed on the front side. If you close your front side it should help you keep your torso, chin, and eyes moving directly towards the catcher, thus the extension you feel at delivery should feel like it’s “stretching towards the catcher”. That front side flying open is also what is causing you to “stand upright”. You aren’t falling to 3B because you are standing upright, it’s actually the exact opposite. You are standing upright because as off balance as you are if you were to finish the pitch and get “horizontal” you would fall right on your head. I see what appears to be pretty good leg drive and torso flexion, but it is all wasted because your front side is pulling all that energy in the wrong direction.
So, in short, try to keep those shoulders closed