When you are stronger than the other person, there are times when assertiveness is necessary but it is only appropriate and effective in rare circumstances. Usually, a little bit of kindness and gentleness is sufficient. Being kind and gentle doesn't mean having ulterior motives to get what you want and merely displaying kindness and gentleness. This is fake and vulgar. Being kind and gentle is sincere, that you are selfless and truly care about the other person. It's not easy. It's not always fair. But it is the right thing to do.
Keeping promises is hard. It requires us to do things we don't want to do. But keep those promises you must do. Otherwise, do not make them in the first place. But since you have made them, you must keep them, especially if it would benefit all those involved including yourself.