I'm not so sure this was the most polite thing to do--and I'm not in favor of emails for emotionally charged topics, actually.
I also think you're VERY optimistic in thinking that you'll never have to deal with this again.
Your mother's strategy for dealing with her daughter's unhappiness is of long standing--it's clearly been meeting some emotional need for her for decades. She's not going to "flip the switch" because you've sent an email out of the blue.
In fact, she's probably going to feel very attacked, and now you'll *also* have to deal with the fallout from that.
The one thing I had wanted to suggest earlier is that perhaps you *tell her specifically what you need from her* when you complain to her.
I find that when people express their unhappiness to me, it can come in a tone that implies "you should fix this!" And for a mother, that impulse would be extra strong, even if the tone weren't implying that.
And since the thing you're unhappy over is not something she can fix (add to that the trigger of knowing she left you down earlier), she pushes the whole concept away.
If you can decide what it is, very specifically, you want her to DO and to SAY, then you can train her. "Mom, I don't need you to fix it--just tell me you feel bad for me and pat me on the arm. That's all I need, really."
Provide a pathway for her to travel--she may not be able to figure out what it is, exactly, that she's supposed to do with your unhappiness.