English has too many words that sound the same, are spelled almost the same, but mean very different things.
Their- There- They're
To- Too- Two
And we can not always find them in our writing on our own. The reason is that we know what it should say, and so we read it how it should be, not how it is. I suggest having someone who understands the differences read over your work and mark them for you. For example, I had Emily Kopf read my manuscript of Bonded Souls (Thanks Emily!), and she pointed out that I tend to use 'surly' when I meant 'surely'. There is a bit of a difference in their meaning that completely mess with the story.
Actually, it is difficult to get right. We are writing stories!! Many of which have whole new worlds and cultures for us to worry about. There is a lot going on. And we all need to stop being embarrassed about our mistakes. So long as we are willing to fix them.