Why cant I just feel normal again? Have i just been down the past few months or is it the start of depression?

okay, so for the past 3 months i've been really down, but i can feel happy, which is why idk if i have depression, im pretty bi polar not like i have it but i have mood swings all the time from happy to sad and their pretty dependable on events like if my friend is comin over im happy then she'll text n say she cant i'll be crying all night and down the next couple days and vice versa from sad to happy. I been doing some research and think i may have atypical depression because you can be happy with it and the whole mood swing thing and i have hypoactive thyroid n its kinda linked to it plus im really shy and try to avoid social situations unless i know the people and i like being alone but sometimes i dont at all and most of the time i cry and am depressed i dont know why and it just makes me more angry that i dont know why, i also sometimes feel like i have borderline personality disorder just without the excessive drinking and cutting, when im with friends im happy or at school but the minute im alone like after school or something im instantly in this dark place, i just want it to stop and every time i feel like im getting better, itll come back, like i have very happy thursday, friday and yesterday and the first half of today and now im just crying and crying and crying for no reason! help! what do you think is wrong with me? i also cant stop thinking about killing myself but i dont think i would ever do it, i just feel it would solve all my problems sometimes, i just hate myself n feel like if i dont love myself no one else can.