The last year I played at 37 years old, a 25 year college coach told my team I was the best third baseman he had ever seen. As a kid, I spent 2 to 3 hours a day throwing a golf ball at a rock wall for infield practice. A golf ball comes at you hard.
Yeah, I get it. I would throw or hit anything any chance I got. At the baseball park, when my older brother played his games, some of us would be in the woods playing "rock-tag" where if you hit the kid with a rock then he was "it". I rarely lost at that. I would warm-up on my pitch back (we reinforced the springs) a few hours before my game, including through about 50 pitches at game speed, then we drove to the ballpark where I'd pitch all 6 innings (in LL).
I was a big kid (at 12), probably 5'9 170, with a fastball (only pitch I threw) probably low 60's. By 15 I was on the HS Varsity team (Soph) probably throwing low 80's (added a slider to my fastball) and worked my way into the starting rotation the 2nd half of the season (4 starts, 2-2), so then when rec-league started up I would pitch every chance I got, even throwing batting practice to my teammates (at near full speed). I dominated that league, only went 3-3 because our offense and defense were so bad (how bad? I lost a 1-hitter when the 1 hit had nothing to do with their run, their run was off a dropped 3rd strike). I think in those 6 (7-inning) games I gave up like 10 hits in 40 innings and probably walked another 10-15, with about 80 K's. But by the time all-stars got there, between HS and rec, my arm was getting tired (dead), not injured, just no longer had the "zip".
I'm not sure I got any better my JR or SR years. In fact, I don't think I did.
I think a good bit of why (I stopped improving, & why I stopped being a good hitter at 12) was just "not knowing" and poor coaching.
As an adult I played some slow pitch softball (mostly Coed, which I liked more). One spring I agreed to be on a fast-pitch softball team, and played 3B. After one game where I'd made some really nice plays at 3b, the coach of the opposing team asked me if I wanted to play (3b) on his Fall-league fast pitch team (but that was when I did Coed w/ my wife & friends).
I really liked the defensive aspect of the fast-pitch (since they could bunt). My thought was if I got to the ball, you were out, not only because I had a good arm, but I also could get rid of it quickly. People focus too much on how fast you throw the ball from 3B/SS, and not enough on getting rid of it quickly. Every extra 0.25 seconds it takes you to get rid of the ball, adds about 10-20mph harder you have to throw it to make up for that lost 1/4 second.