Hi Danielle,
I also started out charging much less than I should have - but that was my choice and my lesson to learn as a business owner. I think many of us have to go through that process, you're not alone
Once I realized I was worth more I knew I had to do something, but I really liked the clients I'd been working with and didn't want to lose numerous accounts.
I started by bringing in new clients at market rate or higher. It took a while but once I had enough clients at the higher rate that I could financially afford to lose other clients, I raised my longer-term clients' rates a healthy percentage. After that I caught them up by raising their rates again each year about 15 - 20%.
Doing it this way took some time but all of those clients are still with me five+ years later (and I'm charging 10 times more now) - so it meant less disruption to my business/income and showed my clients that they were an important part of that business.
Yes, you are in business to make money - but loyal clients are an invaluable part of the income equation. I think that raising rates more than once or twice a year can leave clients concerned over when the next rate increase will come - and it's no longer a stable business relationship when that happens.
I know it gets hard to send the invoices when you're thinking 'I could be making $XX more per hour...' To help address that right away I'd recommend sending notice of a 20-25% rate increase that will take effect in 60 days. Or, if you don't mind losing these clients then you can safely raise your rates with them to whatever you want to charge now. If you want to ensure they're with you in a year or more, take the patient approach.
Good luck with whatever you decide! I'll be interested to learn what you do and how it goes for you.