At every moment of your existence — any moment in which you are mindful, that is — you have some degree of choice about how you feel.
You can let go of a negative emotion, and you can find a more positive response.
For example, when someone is being awkward and obstructive, it’s easy get angry. If we do it often enough we can establish a really strong habit of getting angry when we’re frustrated. But often it doesn’t help us or the other person. (Sometimes it does, but you have to use your judgment and be very wary of anger). But even when you are angry you have choice. You can let go of your anger and find some softer, more appropriate, and more productive emotion — like patience, or forgiveness.
The more awareness you have, the more you realize that you have these choices.
In the fourth stage of the practice (we’ll get to that later — it’s the stage when you cultivate Metta towards someone you have difficulties with) this is what we do. We call that difficult person to mind, we experience the discomfort that comes with the thought, but we choose the emotional/volitional response of wishing them well, rather than our habitual response of getting angry.