In addition to the advice that’s offered, sometimes a pitch is just not your pitch. For whatever reason - lack of physical endowment, temperament, coaching in various forms, and so on, only solidifies the fact that, if you use a certain pitch in real time you’re going to get lit up.
I would suggest practicing this, or any other pitch, at a practice session only. Take the time to figure out what exactly it is that your trying to accomplish. Do you want the ball to approach the plate slower? Then it’s the use of a grip that takes away velocity.
The only drawback to a slower pitch, regardless of how well you sell it in form and delivery, are other factors that compound the chance of the batter nailing the daylights out of it. But then, that’s for anther topic.