The same situation will happen if you don't create a business plan. While you might think that you can keep everything in your head, your results are likely to be disappointing.
The business plan process helps you think through all parts of your business, establish a solid foundation, and puts you in a position for business growth. The parts of the business plan that you can't do or are hard for you, are the areas where you need professional help. A plumber building his own house hires experts in roofing and other unknown areas. You need to approach your business the same way.