Why do I Have These Mood Swings?

I'm 17, and I regularly have break downs where I just don't want to talk or be with anybody and I feel very sad, but for no particular reason. It usually lasts for one or two days and ends after I cry a lot. Once it lasted for a week (I skipped school and stayed at home), and I didn't talk about it to anyone for a few days until my mum discovered and helped me out. I felt suicidal and I self-harmed. But this happened almost 2 years ago. I've been generally feeling ok now, but I still have those mood swings I explained in the beginning, which can be very upsetting. Could you explain to me the reasons of it? Thanks for your help.

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