I try to ride the waves as best I can. I usually get a couple okayish/good days followed by a couple bad/awful ones. It's a rhythm of the mind, proven to be there pretty much unrelated to my external circumstances.
As obvious as it might sound, it helped a lot to actually start seeing it as such. Instead of just seeing it as constantly falling into pits of despair, trying to get out of them in my "better" days, and then each fall just proving how broken, weak and helpless I was. I you don't see the sadness as a baseline or a drop, nor the okayishness as a should-be state one you should be strong enough to mantain, and you instead see the two as equal parts of the cycle, like moon phases or the ying and yang, waves that simply must come and go, to me at least that helps, it gets easier. Easier to take it easier on myself too. I don't dread the falls, I expect them, and expect to move through them.
That's all very buddhist and shit, in reality the bad days still suck and there is little way around that but through them :P They make mantaining motivation and keeping up with basically anything very hard. But I can try and rush things whenever I feel barely good enough to do so.
And all that is of course just for my particular case. I don't know what kind of mood swings you deal with, I hope they are not completely controlling your life, they might be there for a long time, they might not, but there are definetly ways to make them more manageable, be it meds or meditation or techniques or good people around you. So in any case I can only wish you good luck there and that you get easier times your way too. Big hugs <3