I've been reminded of this time and time again throughout life, but especially in my professional life.
For example, at times it shocks me when parents tell me that their child looks up to me or let me know how much of an impact I've had on their child. I've even had a few parents (and a couple of the kids) let me know that working with me has influenced them so greatly that their child now wants to work with people like I do when they get done with college! I can't tell you how much that means to me. It's shocking, but still is a pleasant surprise each time that I hear it. You don't know these things until you hear them direct from the source.
The overall message here is if you're doing great work, then keep at it. I'm sure you're positively impacting those around you to a greater degree than you think.