cfmp - you sound a lovely person, and deserve better treatment.
Long friendships can sometimes end unexpectedly, like long marriages. And it hurts - a lot.
Unless this friend has depression or early dementia, she might have lost interest in the friendship a while back, and not know how to pull away. Some people need an excuse to pick a quarrel, in order to make the break.
A loyal and kind friend who is mentally healthy would never punish their long-term friend over such a trivial mistake.
You could show respect for yourself, and stand back from the whole thing for a while. Not to punish or hurt her in return, but to see exactly how the land lies! If she is a genuine friend and has over-reacted to you, she will miss you and feel ashamed of her treatment of you, and may want you back.
I once had a long-term friend that made a hurtful mistake in our relationship. (Slightly worse than out-of-date chocolates!) At the time I wouldn't forgive her. She accepted my rejection - although she must have been very hurt. She maintained her self respect, and stayed away from me.
However, a couple of years went by, and I had missed her very much. I wrote her a beautiful letter and apologised for my over-reaction of two years before. She graciously accepted. We met up, and she apologised in return, and explained to me, from her point of view - why her mistake in our friendship had occurred. Our friendship renewed, and at a deeper level. We had new, mutual respect for each other. She died last year of a dreadful illness, and I was so glad I had re-connected with her, and was a support to her at the end of her life. I would have felt such guilt hearing how she died, and remembering my rejection of her. We were friends for a total of 47 years! (Not counting the two missed years.)
cfmp - You've had many excellent words of advice. You can choose which advice suits your own circumstances the best!!