Why do i get sudden and ling periods of sadness and depression?

Many factors: No easy answer to your question. If you're having mood swings which are prolonged or sudden then, this could be due to specific situations triggering psychological downturns in your mood or there could be an organic reason such as a hormonal deficiency such as thyroid disease or adrenal gland problems. Also, your menses may have something to do with your moods. Best option is talk with your doc

DEPRESSION: Get an evaluation from a psychiatrist to help identify what depression means to you and whatever else may be bothering you. Consider recommendations for medication if any are given. Get a referral for a reputable psychotherapist to speak with. Meds and therapy combined are most optimal; no just one or the other.