My suggestion is to stride just a little further, drive more of your weight down on your stride leg/foot, exchange your glove shoulder with your pitching shoulder - drive that pitching shoulder into your backstop, drive forward with your head more into the pitch.
All of the above will decrease greatly your tendency to "walk" down your mound with your body following the pitch. When you "walk" your pitch, you're giving up an awful lot of power and other qualities to your pitch that your sending.
Your pivot leg also "whips" up and sends you Peter-Panning sometimes off to one side. Again, that leg action could be the result of your upright posture and other issues.
Here's a technique that I use to use. I'd have the pitcher take the inside bill of his baseball cap and stick a piece of duck tape on it, right in the center under the front rim portion of the lid. Then I'd ask him to rest that vision of the duct tape right on top of the backstop, from the beginning to the end of his pitching cycle. I's also put a patch of duck tape on the pitching shoulder. When the pitcher when through his complete delivery, that image on the inside center of his lid would keep his head into the pitch, and the patch of duct tape on his pitching shoulder should be visible just about under , or near, his chin after release. For some, not everyone mind you, it worked beautifully.
You mentioned " where your buddy coaches," in your post. Please don't use my suggestion to confront another coach and his/her that's helping you pitching. That's not a good thing to do, and basically disrespectful