Chrysippus is quoted by Galen and Cicero in making the following determination about fear. The issue with fear is not that a present thing is evil but rather that an evil thing is present.
This is a very important distinction. We may find ourselves amidst an emotion or passion, and try to coach ourselves that a present thing is indifferent. But, and I think most of our experiences bear this out, often this kind of self-talk fails to ameliorate the impassioned state.
We are not likely to fix every passion in the moment this way. This make sense when we look at the distinction presented above. Since we are dealing with the impression that an evil thing is present we are dealing with old assents.
These previous assents of which items or classes of items are good, evil, or indifferent have been built up and reinforced by us for decades.
What we are doing with the self-talk is very slowly building up new assents. We’re changing our storeroom of classifies objects and classes. We do this unit by unit, time after time. It’s a slow and laborious process.
This is why the regimens of Musonius, Epictetus, and Marcus are firmly grounded in habit forming behaviors. They knew that the change was not immediate.
We cannot (yet?) remember at every junction to reevaluate our goods, but we can build habits in the interim.
Mountain Stoic 
Yes, our prior belief about what is good, bad, or indifferent is a large part of the issue. With fear, we may assume that something happening is terrible and shrink away from it, when the reality of the situation may be different if we take time to reassess. This is helpful in morning meditation, to consider exactly the nature of situations we can expect will arise later that day or during the week, or so I have found.