Frustrated is an adjective that describes feeling disappointed and unsuccessful. If you try to fix your car for hours and hours but it still won't work, you will feel frustrated.
People become frustrated when they fail to achieve their goals, both large and small. If you are constantly frustrated, it can build up into anger or bitterness. People sometimes need to vent, or express themselves, when they are very frustrated, in order to let it go. But if you can't shake that frustrated feeling, maybe a change is in order — a new job, different friends, maybe even a cat to remind you that calm is good.