Wow Paige, your answer was very wise - and it sure sounds like you are coping with your illness well, kudos to you!!
MKate ... I wasn't diagnosed until recently, but I believe I've had the illness since I was the same age (or younger) as Paige and your boyfriend. It *is* a really hard place to be in - being an adolescent is hard enough, without whacked out chemical imbalances added to the mix. I definitely felt, at that time, that there was NO way in hell that anyone would understand my suffering, and remember was dead surprised when my Mum did seem to understand. Even now, when I'm in the throes of depression or mania, I just feel so remote from everyone else that I can't fathom them understanding what it's like to be me.
And yet, as Paige was saying, it is possible for loved ones to understand it to an extent. Taking an informed interest is a really good way of showing that you are willing to put effort into learning about bipolar disorder, and even teaching your boyfriend things he didn't know about it as well. My partner has been excellent, he knows he doesn't fully understand the disorder because his highly mathematical mind finds it hard processing the chaotic nature and unpredictable course of the illness, but he has latched on to what he can predict - how I act when I'm in a certain mood, and he can even read the signs of an approaching depression or mania. Which is impressive, he even does this despite us currently being in different cities! He has also learnt to separate Bipolar Tam from Tam, and knows what not to take personally. This is hard to do, and takes time, but it is an important skill that is needed to love someone with bipolar disorder.
As for dealing with your partner:
1) Be careful in the way you say things - simple word choice can make all the difference. Here are some suggestions: Instead of saying things like "why do you keep drinking coffee when you know it makes you feel unwell?" or "You drank alcohol again! Why did you do it? That's really stupid!" phrase it like "It really hurts me when you do that, I care about you and want the best for you." In your particular situation you might want to add "Please be honest with me about what's going on, nothing you say is going to make me love you any less. We can work on this together." Reassuring them that you love them regardless of the illness is vital.
2) Paige is right - sometimes bipolar people do need a little bit of emotional distance while they are manic. Physically, you'll need to be there to ensure they don't hurt themselves or anyone else, but emotionally, you need to put on your Thick Skin Armour and not try and talk to them about issues relating to a) your relationship b) their illness or c) anything of a serious nature because their answers will be skewed and/or ones you DON'T want to hear (then retracted once the mania has subsided but by this point, the damage is done).
3) Anger is a common symptom, usually associated with manic or mixed episodes. As above, definitely take emotional distance from your partner's anger. Definitely give them their space, and do not provoke any further anger. In less severe mania, it will burn out in its own course, if it doesn't - seek medical attention. Also (when he's not angry) talk to him about anger management techniques that he can prepare for when he is angry. For me, number one - I isolate myself so that I can't be tempted to say terrible things. I hide in my room and just scream inside my head at whoever or whatever has made me angry until it dies down, or I write it on a piece of paper in BIG HUGE LETTERS WITH LOTS OF EXPLETIVES. That helps too.
Well, enough rambling from me, hope this has helped!!

Best of luck, and if you have any questions at all, we are here to help!