The phrase "fed up" could be substituted with several other synonyms. Some of these words to express that you are fed up could include: frustrated, bored, irritated, annoyed, exasperated, and vexed. Furthermore, you could use words like sick and tired, jaded, disgruntled, weary, and disillusioned to describe your feelings of dissatisfaction and exasperation. Using synonyms can help you to better express your emotions and can make your communication more effective. It is essential to recognize your emotions and express them in a healthy way, and using synonyms is one way of doing that.