A flat surface is good for fielders, but not pitchers.
Take a look at any infielder, like a shortstop or third baseman fielding a ball then sending that ball to first across the infield, and you'll see your posture. Besides the leg lift, your body's posture is that of an infielder.
An infielder starts his/her toss off the back leg, then completes much of his/her toss (not pitch) while somewhat centering his/her balance. In fact, a good infielder's throw can have stability by planting the front and back feet so a line can almost be drawn right down the center of the player just prior and during the throw.
Pitchers on the other hand don't throw, we pitch. In doing so, we follow what Roger has pointed out. We pitch off our front leg, exclusively. All our presentation is "prepped" toward our forward motion for that final report off that front leg.
Obviously, there's a lot more to this than I'm narrating here. In essence, if you maintain your composure to drive forward, keeping your upper body in the pitch .. and down, exchanging your glove shoulder for your pitching shoulder, NOT popping up at the end of your pitching cycle, you'll be able to train yourself to repeat this part of your pitching cycle and deliver strikes, a greater percentage of the time.
Find yourself a gentle incline somewhere in a park or anywhere else with the landscaping that I just described. Slowly go through your motion, like you did here in your video. Concentrate on what Roger and I suggested. Take video and present it here. We'll take it one step at a time and explain how you can take advantage of that athlete's composure that you bring to the field.
A small suggestion - relax a bit more. You seem a little tense and stiff in your video. I know sometimes taking video of yourself does that, the tense and stiffness part.