Is this a mental illness or could I have some kind of hormone or physiological problem?

I am often really tried and feel depressed during the day time but as soon as the sun sets I feel wide awake and have lots of energy. This makes it hard for me to fall asleep. I have been this way since I was about 13. I'm 27 now. I feel the cold really really easily. Other people will tell me it's not that... show more I am often really tried and feel depressed during the day time but as soon as the sun sets I feel wide awake and have lots of energy. This makes it hard for me to fall asleep. I have been this way since I was about 13. I'm 27 now.
I feel the cold really really easily. Other people will tell me it's not that cold or they will be sitting there in a t shirt and I will have a thick coat and a heater on. I thought I might have a thyroid problem but the doctor tested 1 of my thyroid hormones along with some other things and they didn't find anything unusual.
I have really dry and itchy skin.
I have mood swings where I will get really angry or agitated for no apparent reason or get upset over something really minor. I can feel hyper and extremely tired at the same time.
The doctor thinks I definitely have a mental illness and prescribed me a mood stabilizer. I don't like to take it because it makes me gain a lot of weight and it makes me sleepy if I take regularly. But I wonder if there is some other explanation for my symptoms that is not mental illness. If I could have a hormone imbalance? Or even lacking something in my diet?
I eat a balanced diet but I maybe don't eat as much vegetables and fruit as I should.
Has anyone experienced anything similar to this?
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