Tennis is like body building in the sense that you can only improve so quickly. In body building terms, you can only bulk up so fast. A body builder does thousands of reps for such incremental gains just how a tennis player must hit thousands of balls before any noticeable improvement.
Development is a long, arduous process, so you must 1) adjust your expectations accordingly and 2) take the approach of sculpting a piece of art. You can’t get a six pack overnight - everyone knows that. But if you keep chipping away at that sculpture, you’ll end up with something beautiful over time. However, if you don’t start now and put in the work to chip away at that sculpture, it doesn’t matter how much time passes. You’ll have that same ugly hunk of stone.