One of the great things about being in business in the 21st century is that so many of our interactions with other people are written down. E-mail, blog posts, tweets, etc.
It used to be a big deal to put something in writing and send it off to someone else. And so there was a lot less of it than there is today.
I mention all this because whether you realize it yet or not, in the course of doing your work as a solo professional, you're going to get written praise from happy people. Sometimes, dare I say, even fan mail.
An e-mail from a happy client, telling you how helpful you've been. A tweet about something you wrote in which you're referred to as "genius." Maybe just a thanks from one of your newsletter subscribers for brightening their day.
Whatever it is, take my advice and print it out. And save it. Keep it in a pile, somewhere close at hand and look through it every once in a while. It will make you feel good on the bad days and gooder on the good days.
I've got my (massively huge) pile right here. I think I'll go read a few right now.