I am not going to lie to you. If you did a really good job of developing your character they will take on a life of their own. It makes no difference if you are a planner or pantser at some point this is going to happen to you - if it hasn’t happened already.
As a pantser, this can be great - so long as you keep your end goal in mind when you’re writing. Characters that take over are a blessing if you can stay out of their way long enough to be able to document their adventures. Sometimes you may have to poke them and prod them a bit to keep the story heading more or less in the right directions, but never try to force a character like this to do something that they do not want to do. Generally, if your character has taken on a life of their own then trust them to act in the story’s best interest.
However, you can run into trouble if your characters try to take over, but aren’t well developed enough to really hold their own or they are a Mary Sue. I’ll write more about Mary Sues in a bit.
As a planner, this can be terrifying - especially when you’ve spent all this time plotting out your novel just to have your characters hijack the story and take it somewhere unexpected. If they are behaving (more or less) - congrats! You probably considered who your characters were prior to planning your novels. Of course, that doesn’t prevent any new characters from just popping up and demanding your attention.
If you do have a character trying to derail your novel the most important thing to remember is that sometimes you need to follow that muse. The worst thing that could happen is that you end up with a terrible dead end side story that you can delete when you edit. There is absolutely no reason why you can't go back to the point of derailment and follow the story down a different path this time. These are still words and, better yet, if you are drawn to write this, if it is easy to write that dead end path than do so because those are words you can count. And that best case scenario? You end up writing something even better than you had planned. Chances are, though, you will be somewhere between those two extremes.
What do you do about the Mary Sues?
We all end up with these at some point in our writing career. They do not make for the most interesting reading material, to be honest. If you are halfway through your novel and you are suddenly realizing that one of your main characters just might be a Mary Sue then you’re going to have to do some quick character fixes. (Unless, of course, your story is flowing just fine. Then by all means keep writing and fix that character later).
Keep in mind, that no one is perfect. People have flaws, mental and physical. We struggle, we debate, we worry, we get angry for what seems like no reason. Remember your character isn’t just a character - it’s a person. Make sure that this person you created has failings and flaws. Has things he/she/it struggles with and things that they are good at. Make them round and add a few flaws. If you have, after November, go back and fix that first part. Or if you are really mad at your story and nothing is working out for you there is no reason that you cannot ignore everything you’ve written until now and start back at the beginning with your now, more well-rounded character.
In the end, no matter where you stand, the point here is that sometimes you just have to follow the story, and your characters, where they take you. If you try to fight it you may all too often find yourself blocked in what to write next or just plain uninspired by your own story.