The first thing you can do is accept their existence. Things aren't always rose-colored in our minds and in our environments, so being in a 24/7 joy would be abnormal. The gravity of the feeling depends on the person though. As does the frequency.
One could stay there but I'm not a believer of easy solutions. Next step would be get over it. But not in the form of ignoring it. Getting over it, for me means learning to live with it and not letting it get you out of track. Having violent mood swings feels like walking on a thread. One false step and you're hanging upside down. But, with the right push from the people around you, you can do it.
As for disappointments, they've been a very frequent occurence in my life for the past three years. I could even say that the past year has been a series of massive discouragement. These might be tougher than mood swings, because they teach you not to expect anything, in order not to get hurt. And what life, what struggle can exist without expectations?
Anyway, I'm done with the foolosophy for today. Let's catch a glimpse of the weekend.