A better way of saying this is "The secret of health is not to mourn the past, not to worry about the future, not to anticipate troubles, but to live in the present moment wisely and earnestly." – Buddha. This is what they mean by "One Day at a Time." Don't endlessly grieve over something difficult that may have happened to you. Don't be worried about whether some goal you have will work out exactly the way you want. Just make sure your goal is pure, unselfish and loving and do what you can today to head in the direction of the goal.
Redirect any thoughts of worry (won't get my way in the future) and regret (didn't get my way in past) to thinking and doing in the NOW. The way you redirect your thoughts is to visualize a big red stop sign, then say to yourself, "STOP!" Then, ask your Higher Power or Higher Self to, "Please let me focus on the now." You can also call a support person and share what is bothering you and ask the support person for advice on how to focus on doing the next right thing.