Knowing what you don't know is an extremely important part of being good at whatever you do. It's easy to get lulled into feeling like you're "good enough" and thus don't need to keep striving. I have caught myself doing this lately. The only way that you can really get in touch with knowing what you don't know is to push your boundaries. Try things you've never tried before. Stay in that unfamiliar feeling when you don't know something. It's uncomfortable, but necessary. Don't think you know everything. Use that unfamiliar, uncomfortable feeling to hone in on what you need to work on.
I came across this unfamiliar feeling recently when I was trying to turn something I made into a jQuery plugin. I quickly realized I don't know as much about JavaScript as I thought I did. I could have just pretended I didn't feel that, but then I'd just be kidding myself. Instead, I looked at it as an opportunity to give myself a good foundation in JavaScript and that's where I'm currently at.
Do it, then do it right, then do it better.
-Addy Osmani
This quote is perfect. I can do the first part (Do it), now I'm working on doing it right. It feels good to always be learning and bettering yourself.