Mentoring is very important, but I think it's best coupled to an open source workflow that forces you out of your bubble. You can learn very effectively by looking at other projects online and then creating your own. With package managers like npm, you can easily create and publish tiny utilities, whether they're standalone libraries or plugins, and share them with the world, get feedback and iterate. In some sense, this is just an open, asynchronous form of mentoring, and one that works surprisingly well.