I'm confused. You say you owe her an apology but meant every word you said, so what exactly is the apology for? Are you apologising for having your own opinion, or the way you expressed it? Is she not equally at fault?
I'm with Mustard -- why, exactly, do you feel the need to apologize?
You said in your OP
Anyway, I handled myself really well until one point where I just completely lost it.
By the way, I did apologize before we left that night, and we left with hugs. But it wasn't anywhere near being able to repair the damage.
Then in your next post:
It is confusing, Mustard. I did mean every word, I said. And the weird thing is, although my heart was beating and my voice was shaking at the height of this, I did not poorly or aggressively express myself.
The bolds above . . . the point where you completely lost it but you did not poorly or aggressively express yourself, and you apologized along with leaving with hugs.
What damage was done? That wasn't resolved with a hug and apology when you left?
I believe that apologies are never too late.
Your friend still cares about you if she sent you a Christmas card. If you love your friend and wish to maintain this relationship you should figure out what you are specifically sorry about and apologize. If it was your delivery, you demeanor, your words. Maybe in your anger you dealt her a low blow and regret that.
I agree with Easter Hat . . . even though it sounds like (to me) that you have nothing further to apologize for it is obvious that you still think an additional apology is necessary.
Is she giving you the cold shoulder? I suggest that (after you've figured out what you're specifically sorry about
) you give her a phone call to discuss the weather or whatnot. If she's open and friendly then I suggest that it's all under the bridge and forget about it.
If she feels cold to you then you can bring up "Are you still mad at me about that night?" and ask her "Why?" Listen to her thoughts before you reply.
A letter is one-sided. A phone call/real life conversation (invite her out for lunch?) is more personal.
Just my thoughts.
Alicia posted while I was typing.