Deliver promised milestones, collect wins, tell clients as milestones are delivered that you were happy things worked out for them and, since your business is maturing, you're switching to day rates to maintain your ability to focus to be able to focus on their projects. Clients unwilling to pay your day rates will be serviced as your schedule permits. If you're burned out, then that can be the first day after never. Your health and happiness get priority.

I'd probably also take the opportunity to raise rates.