I have heard that it promotes healing to hold positive thoughts in mind. Is this true?
It depends what you mean by healing and what you mean by positive. Your mental and physical life is closely intertwined. The repetition of soothing phrases in your mind can encourage an assured attitude. As a rule, a relaxed attitude promotes healing. But you cannot force this. There is a lucky few that naturally hold an optimistic attitude no matter what. But there is no way to rapidly change yourself into such a person.
Many have grossly overstated the power of positive thinking. We think you are unlikely to become a millionaire just by thinking positively. Similarly, you are unlikely to cure yourself of serious disease with so-called positive thoughts alone. Due to exaggerated claims, some people with serious illness strain to hold positive thoughts or images continuously. They are told that this can help their disease remit. Discouragement or guilt occurs when this does not happen.
There are other potential complications with this approach. It is tiring. You may miss some important prompts to action by trying to ignore thoughts that you label negative. Finally, frustration and demoralization may arise when you find that you cannot control completely, the thoughts that occur in your mind. There is little in these consequences that will encourage any type of healing.