hey rachie, yes, i've had those moments. my major dx is bipolar with hypomania, so before i got stable with meds, i could swing from the rafters one moment and be in a dark hole the next. i'm what they call a rapid cycler (and that's not on a bike.)
before my diagnosis i could have a really upbeat day...i was productive and happy on those days. (this was unusual for me cause most of the time i was very depressed). i'd go to bed and wake up in the morning severely depressed. it was very confusing cause all i did was go to sleep! so i got frightened when i had a good day cause i knew the "darkness" would be following it.
my T later explained to me that for every "upbeat" there would be an equal (in intensity) "downbeat". i thought how will i ever deal with this? for me meds and therapy helped me to a better, more balanced life.
i hope you can find that balance too.