Hello, I'd like to help you with your question.
From the symptoms you describe, it sounds like you might have Bipolar
disorder, possibly Type II, although it is not that common in your age group. But because you are having periods of extreme highs and periods of extreme lows, Bipolar disorder should be considered. Extreme highs (feeling good, on top of the world) and extreme lows (no motivation, depression) are signs of Bipolar disorder. Therefore, ruling it out is a good idea.
Your first step should be to find a professional who can provide an accurate diagnosis. If you have been seeing a therapist, you should have had a diagnosis. But if it was not recent, then you may need another evaluation to be sure of what might be going on. If you are diagnosed, there are medications that can stabilize your mood and help you feel better.
Another possibility could be menopause. Feeling extreme mood swings is not uncommon and is due to fluctuating hormonal levels. The best way to tell if you are in menopause is to ask your doctor for a hormone blood level test. They can tell from your results if your symptoms are caused by menopause. If so, your doctor can give you treatment options to help you feel better until you go through menopause.
I hope this has helped you,