Going beyond my other post about being a watchman at the gate, the ulimate choice would be to have a quiet mind. A quiet mind allows a person to be in the present, where the past and the future aren't around. This is where peace really exsists. This seems to be a problem with people with mental helath problems.
Think about it, you were having a good day and then you have the thought of something in the past. It is not going on right now, but happened in the past. You have to be a watchman at the gate and then shoot for a quiet mind and live in the now.
It could go the other wa and think of a worry in the future. Again, the future isn;t here, but in the future. Bring yourself back to now and be fully aware of what is going on.
This is what the ancient meditations of the past did. Cultivate the mind to stay in the now and when it is thinking again, especially in the past or the future, bring it back to the now.
This takes practice, but it is worth it to have the peace. The more you do it the more it will become normal.
Staying in the now is the best, but if you have to think, then think pleasant and Godly thoughts. Stay away from the negative.