Most people can understand that a thought can make you feel sick. For example, if a student before their exam has a thought, "Oh my god, Iam going to fail", he or she will feel sick, and sometimes, if it is severe enough, he or she may even feel like vomitting. So if a thought can make you sick, it is reasonable to assume that the reverse is true as well. A thought can make you feel better.
This is the power of a thought!
This is one of the essence of Cognitive Therapy.