Being able to control your thoughts is a huge deal. It is a very important skill indeed.
As long as you are not dwelling on them though, I wouldn't worry about it. Concentration is key. Picture it like reading a book. While you are reading, you are completely oblivious to what happens in the external world. You are absorbed. That's the kind of concentration you need. A flash of a second worth of thinking won't do anything. In fact, I am not sure how much 15mins of meditation does either (perhaps it is the prolonged exposure over time). You really need to be infatuated with your ideas!
There is another issue here though. You already have the results of being absorbed with your previous thoughts until now... Have you been a worrier? A negative thinker? You gotta break those patterns. What is in charge now, is what you've been absorbed with in the past. Good news is all that can change. Change your focus of infatuation, and the rest will take care of itself.
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