"Letting it go" will only work if you can evaluate the worth of the situation. Should you insist on being "right", or forcing something on someone, or insisting something is done your way? I look on it all in the light of the relationship I want with the people in the situation. In parenting. in marriage, in friendship or in business if you value the relationship then you must be able to evaluate the situation and ask yourself, "Is my behavior going to bring us closer together or push us further apart?"
Once you take the time to answer the question and evaluate, you will have a good idea of how best to handle the situation.