People with high self-esteem are not always happy. The difference between a person with high self-esteem and a person with low self-esteem in terms of emotions is instead the way in which they deal with their negative emotions.
You cannot prevent negative emotions from coming from time to time. Actually, negative emotions are not “negative”. They come to you to help you notice some unfulfilled need or warn you of something in your life that needs to be changed. Negative emotions help you to build a balanced, healthy and happy life. Negative emotions should therefore be listened to rather than ignored.
Many people with low self-esteem try to ignore their negative emotions. They may have learnt through their childhoods that negative emotions are bad and should not be expressed, or they may simply not know how to handle their emotions. People with low self-esteem often translate having negative emotions into being a bad person. They assume that if they are feeling sad, angry or scared it must be because they are either doing something wrong or they do not deserve to feel differently.
People with high self-esteem listen to their emotions without ignoring them. They do not equate feeling bad with being bad. People with high self-esteem learn from their negative emotions and take active steps to make themselves feel better (for example by making sure that they get their unfulfilled needs met).
To keep your self-esteem high you thus have to accept your emotions. Ask yourself what you need in order to heal your wound or to find better balance. If you need social support, ask for it! If you need to slow down and relax, do it! You may simply need to have a good cry to offload some pressure, and everything will be fine again. Welcome your emotions and listen to what they are trying to tell you!