Mixing your own music can work but more often than not you’ll run into ceilings as far as quality. One of the main reasons to work with a professional mix engineer is for their expertise and experience. It's a very creative and technical exercise that requires a large understanding of the science of audio.
Beyond the know how and expertise, you need a tuned listening environment to properly perceive the audio as it is rather than how it sounds at your house. There are reasons why studios spend thousands of dollars on professional acoustic treatment, audio gear, and listening environments. When you're trying to get your music to sound its very best, you need the right tools to make a record compete as well as translate across all listening environments. Having a neutral starting place for mixing your songs helps ensure they will translate to headphones, car speakers, and especially at the club.