Working for yourself as a marketer and copywriter means also being your own editor (and accountant and HR department and...). I touched on this lightly in a recent post, but I think editing woes are far from uncommon. I'm always looking for new tips and tricks to better edit myself. If I can print something out and edit with a pen, I will. If I can look at something in a different medium, I will. If I can sleep on something I've written and then return to it a day or two later with fresh eyes, I will. Even sitting in a different chair or a different room helps shift my perspective and see things I otherwise missed. Reading aloud helps me as well.
All of that said, of course, I'm not perfect. I cringe when I spot an error in a published piece. I'm always looking for ways to better my writing and recently discovered Grammarly. It's an app and desktop plugin that highlights mistakes you've made and makes suggestions for correcting them. It's a little like having your English teacher looking over your shoulder and clucking her tongue as needed. A red line pops up under a mistake, and you can then view suggestions for fixing it. Sometimes the rule doesn't work and I use my judgement to override it, other times I'm grateful for the direction. As with spellcheck, you're still responsible for your writing, but it will help you catch some errors and think more carefully about what you're saying, and how.
Grammarly will also send you a weekly progress report with tips and stats that highlights what you're doing well and where there's room for improvement. I'm currently struggling with the incorrect usage of the progressive tense, but my vocabulary is more dynamic (unique/total) than 72% of Grammarly users.
The basic version of Grammarly is free, but you can pay for an upgraded version that identifies more complex issues as well.