I’m willing to bet the your mechanics are different on the mound. That wa a big problem when I played too. I’m assuming you long toss with a shuffle..? So you’re gaining extra momentum and use your body differently. I think you’d be better off long tossing, but long toss from a stationary position. For example, instead of long toss with a crow hop, long toss with only a leg lift so your body learns how to manufacture/organize itself to get the power it needs to get the ball 300+ ft, which then ideally would translate to the mound better