I'm sorry that he is so volatile. I don't really remember being angry and lashing out, but each person's process is different. It is...counterproductive. You will never completely understand what is happening inside his mind, just as he will never know completely how much he is hurting you. It's just the way it is, and for him to resent that is illogical.
Try to let his temper roll off you; this is not about you. You are simply a convenient target. And don't let him bully you into feeling guilty or rearranging your life to suit his whims/moods/preferences. Being understanding only helps up to a point. There is no reason you should cater to him if he will not take responsibility. If he knows he is bipolar and he knows he needs medication to control it but chooses not to, he is responsible for the consequences.
I feel fantastic now. I am calm, clear-headed, emotionally stable. But that doesn't mean I'm "cured!" Counselors tell you as soon as you are diagnosed that bipolar disorder is a permanent condition that may or may not improve over time. With age your body's chemical balance tends to improve and some individuals can learn to control the situation without medication. But that needs to be carefully controlled and guided by an IC.
I don't want to shred your letter to pieces; you said what you felt. But I think there are some things you want to consider in the future.
Instead of "worried about you condition" you might say something like "I notice you've changed. I hope you are doing OK." This says that you are observant, but is less involved. These are his choices and he may not suck you into his self-destructive nonsense. If you "worry" about him (or any other emotionally involved response) it gives him the opportunity to attack and blame you. "Well it's your fault because you don't care enough. If you were really worried you'd help me and be nicer when I have a bad day."
Do not address how he is feeling, IMO. He will just tell himself you have no idea how he feels and how dare you make assumptions. Never use the words "I'm sorry" either. Even if you mean to sound sympathetic, he may take it as an apology, again allowing him to blame you. Not your fault; not your problem to fix. He knows the solution and he has to choose it.
Unfortunately, the rest of the message was not so good. In this respect typical DBing is probably a better course. He doesn't want to hear ILY or "I'm troubled too." He isn't listening and it will be likely to make him withdraw further.
I'm not sure his emotions are "stronger," but they are almost never under control. He'll tell himself whatever it takes to stay where he is. He is gaining something by not getting treated. Whether that is not having to face his bad choices or not feeling capable of coping with reality, or something else entirely he is deriving some sort of benefit (in his own mind). Please, try to let it go. It is in your own best interests to do what is healthy for you. Letting his sickness taint your life just makes the thing so much worse.
undefeated 24 H 24 S's 4, 2, 1 M 5 yrs
"Most of the important things in the world have been accomplished by people who have kept on trying when there seemed to be no hope at all." ~Dale Carnegie