You can think yourself healthier. Scientists have just published another placebo effect study, this time showing increased vaccine effectiveness. The people who did positive thinking exercises showed increased antibody production after 14 and 28 days.
The inverse is that you can probably also think yourself sicker, though for ethical reasons this has not been studied.
Make sure you have a technique for getting yourself in a better mood. For some, it can be uplifting music. For others, it might be a walk in nature. Spend a few moments identifying something that energizes you. And then do it today.