Well, I don't really have a full answer for you dearie, because everyone, including me, has mood swings. I would suggest just working on recognizing your emotions and using positive selftalk (that's how someone once said it to me) to convince yourself To feel differently. If you feel horrible then remind yourself of good things...like listen to your favorite music or whatever. If you know you're being sort of bitchy then take a minute to calm down. It's always a race to control yourself...that's the sucky part about being human I guess.

well... i have a few tricks that work for me. i have certain thoughts and songs which sort of help me get in the mood. so when i am feeling a certain way while i need to be feeling differently (eg. going to a party but feeling depressed or something) i let positive thoughts take over me and listen to upbeat music. although if its a sadistic moment while i am feeling happy, i just think of dark, suicidal thoughts and listen to sad songs. personally, i am able to manipulate the way i am feeling. also helps me blend in with others, otherwise who knows how i'd act lol.

I suffer from this too and completely understand the feeling.

I, for example get a sudden change in mood, and notice that something so small could easily trigger it. Are there things that could possibly be causing these sudden changes? Like any little things that your mood appears to be sensitive to? It's extremely helpful to be talking this through with someone, for me it's been the most helpful thing.

I think most people here will have mood swings.
I have em pretty violently too (was going to the seaside with some friends, was in a great mood but then, to cut a long story short, got kicked off the train for punching a window and then shouting at the attendant who came to ask me to calm down ^_^)
But what I try to do is listen to music that'll calm me down if Im feeling angry or aggressive or usually either sad music (if I'm alone) or happy music (round others) to feel better.
The reason for the sad music is that it'll help you feel better, letting all the emotions out that you've bottled up! Then the happy music to give you a quick boost if you're out with friends so you can be happy with them
Think of it as if you're playing a game of rugby, football, any sport really in a lucozade add! P:
You're gonna get tired, you're going to want to stop and rest, but you need to keep going to win the game, so you take a quick drink of lucozade/powerade and overcome the tired feeling, then rest when you get home!
Hope you all understood that ;D

Is there a sport or a physical athletic activity that you could do every day? If you get he endorphins moving and are more active it might help because physical activity balances out your hormones a bit, and at least for me always makes me feel better/more sane.

Find something that will put you in a neutral mood. Whether is be a piece of music, a scene from a movie or just an inspirational picture, but do not do anything that will fuel a bad mood.

Oh also,
Banana's apparently have a chemical that make you feel good in them
So make a banana sandwich or something