"Comparison is the thief of joy"
Right away the quote seems obvious. Don't compare yourself to others because we're all different and you'll just make yourself miserable. But while this is true, Harland went on to talk about a different type of comparison that I hadn't thought of. He said that not only should we not compare ourselves to others, but we shouldn't compare ourselves to ourselves.
On one hand I don't necessarily agree, as I think it's important to look back to see how far you've come, and to look ahead to see where you want to go, but what Eric was getting at was that we shouldn't waste our time thinking, "I'd play this much better 6 months from now", or "I wish I could go back and play that gig from last year now".
The gist of it was, when you're playing, gigging, practicing whatever, don't waste your mental energy thinking about what you could have done better before, wish you could do better now, or what someone else can do better than you. Rather, put that focus into the music you're playing right now, and make the best music you can make with the skills you have today.