I know you're probably amazing at whatever it is you do but I'm sure there are areas that are outside your depth of knowledge - areas where you aren't so amazing. No problem, that's why you have experts as friends, colleagues, and mentors. If you have the budget, hire them. If you don't have a budget, offer an exchange, or buy them dinner and drinks.
Consulting experts will get you to solutions faster and shorten your conversations with clients. That's more upside for you if you're working on a project or flat rate. If you work hourly it means moving to your next project sooner.
Where do you lack expertise? Now, which of your friends and colleagues have the skills you lack? Reach out to them. Don't worry about being a pain in the ass, you'll return the favor at some point. That's cheaper than going to school or learning the hard way.