No matter how experienced the writer, perfection and human don’t usually go together. Even professional editors and proofreaders can occasionally miss a mistake.

Why do writers sometimes have ‘wobbly’ spelling? The International Association of Professional Writers and Editors offer a scientific reason here: https://iapwe.org/tag/do-i-need-a-proofreader/.

So, there’s no shame in asking for assistance. You’re in good company.