Stuff gets into our heads through --
- Commercials.
- Mass media.
- Interactive media (email, social media, push notifications, text messages, phone calls ...)
- People close to us (family, friends, co-workers).
- Other people we run into or interact with.
- The environment (natural and man-modified)
- Self-talk (the comments, judgments, memories, anticipations, and speculations that spontaneously arise, often without our clear awareness).
If we protect ourselves from numbers 1 - 6, our minds still may not be our own, because self-defeating self-talk can automatically take over.
I personally find it hard to "own" my own mind.
Mindfulness helps. I do it while walking, exercising, doing chores. But with one or more of the mind invaders almost constantly ready to attack, it's a hard slog.