Not every working hour can be invoiced
A 40 hour week rarely contains 40 billable hours. Pitching, admin, revisions, context switching and learning all consume time. If you price as if every hour is billable, the gap comes out of your own income.
Measure the ratio once, then use it
For two weeks, write down how many hours you worked and how many of those you could honestly invoice. The ratio is usually between 0.5 and 0.7 for independent workers who also do their own sales and admin.
Build slack into the rate
Once the ratio is known, stop treating it as a failure. It is a cost of running the business and belongs in the price. Add a profit margin on top so one bad month does not become a personal cash problem.