I was thinking this also. Be alot like a job. I would like to hear from the parents of the older girls that stuck with pitching over the years and how they kept them interested for so long.
One of the keys to keeping my DD motivated to pitch was the fact that she knew her coaches and teammates were counting on her. We intentionally held her back a couple of years, because we thought it would be better to be the #1 or #2 pitcher on a good B-level team vs. the #3 or #4 on an A-level team. When you know you will be pitching the championship game on Sunday if your team makes it, it motivates you to push yourself.