You may be correct that you have the capability and skill to achieve more. If you only occasionally fail to meet your own expectations, then look more closely at what is preventing you from doing what you want. Perhaps there are levels of success to reaching your goal. Avoid thinking of success as all or nothing. Consider that satisfaction in life may come from a quest for constant improvement and not the actual achievement of a goal alone.
People fail every day.
Only underachievers interpret this as failure. Evaluate your situation very carefully. When your unrealistic expectations result in the perception of underachievement, scale down or modify what you expect. Work to develop more realistic expectations in future situations. Setting unrealistically high expectations for yourself guarantees you will feel disappointed.