I have found that worries are energy-draining and doubt is self-destructive. The more you worry about what you can do, the less you can do. Doubt is a useless thing; it only slows you down.
OK, no offense, but that isn't very descriptive advice. Telling someone 'just don't worry' doesn't flip a switch in their brain (God, I wish it was that simple). My advice for dealing with your worries and doubts would be to step aside for a few seconds, take a few breaths, and work through your problems logically and at your own pace. Cooler heads prevail and I have found that thinking about something that brings you joy and laughter helps to ease the stress so your mind does not go into panic mode. That is about the soundest advice I can give. I deal with stress and worries daily and the more I repeat these steps, the simpler it gets to calm down before I work myself up into an anxiety attack.
This is actually quite helpful, to both messages. I could definitely use some time to relax and think about what I can do to make situations less terrible than they are.