This is an old adage. For millenia philosophers, sages and ordinary folk understand on a fundamental level that everything begins and ends in the mind.
We are talking about thinking. Not magical thinking or wishes. This is thinking with action.
It’s making up our minds and acting on it.
Of course this is simpler said than done. If you are fed up with the same negative results over and over again then it’s time to change some small action. One small change will kick in the domino effect. One thing affects another and so on.
Keep in mind it takes about six weeks for any change to become effective. So have patience.
Change demands boundaries and commitment and movement in a gentle way. In a non-judgemental way.
It’s OK if you fall short of your objective. It’s OK if you start again from square one. It’s OK if you re-adjust again and again to get to your objective.
Just start and try. You can always change your mind.