Both of them can be motivators. Psychological research, however, has shown that the best performances take place in an atmosphere of positive energy, often described by the participants as a state of joy. Although negative energy can motivate you to perform, it is more often associated with negative motivation. Negative thoughts and fears usually inhibit performance rather than drive it. In competition, once doubt gains a foothold, it has a tendency to snowball into many negative thoughts, loss of energy and poor performance.
Winners recognize and have ways of dealing with negative thoughts.
If possible, they avoid them entirely. Otherwise, they are able to cope with them rapidly and dismiss them. Try to detect negative thoughts while you are running or thinking about your running. You may wish to write some of them down and think about them later. Most of the others can be turned into challenges or put into a positive perspective.