I've noticed something about my writing, and thinking back over the thousands of books I've read in my lifetime, it's really not that unusual. What am I talking about, you ask? Patterns. Every author seems to have patterns in their writing that all of their stories follow, at least to a certain extent. One pattern that's prevalent in a lot of my writing, especially in the YA written under a pseudonym, is this: girl is in bad situation, girl moves (sometimes to non-custodial parent, sometimes to foster/adoptive family), life improves. I know that sounds really dull, but that's my formula that I can't seem to get away from. Now, out of that I can come up with dozens of unique stories.
How does that work? Well, it's kind of like the romance formula: boy meets girl, boy loses girl, boy gets girl back. Sounds boring until you look at all the thousands of romance novels out there that are unique and great reads. Just because there's a formula doesn't mean all the stories are identical; only the underlying pattern is the same. You can have an endless number of plots that are at opposite ends of the imagination, yet they follow the same basic pattern.
I guess it's kind of like the old saying that there aren't any new ideas, only new ways to write them. Following a formula is the same way. It's the same old pattern written in a new way. And that new way is what makes great books.
Don't worry if all of your writing follows a pattern. Write the story in your heart. Make it as unique as possible, and embrace your own personal writing formula. Changing the formula that works for you is like trying to change your handwriting: yes, it's possible to do, but you're likely to have an awkward mess that doesn't look natural to anyone.