Your statement at the end, about not caring, is a bit Catcher in the Rye-ish. Deflecting hurt by 'not caring'.
So last year he forgot that you celebrate the day you met as he was entering an EA, and in March had a PA. Was this with the same person?
Also, in the midst of his PA, you and he took a vacation on the anniversary of your wedding, and not surprisingly given his PA, it was not a good trip.
Have you discussed his missing this date last year in MC, or elsewhere? If so, it's surprising to me that he would forget again this year.
it isn't surprising that this is causing some triggery feelings. You've attributed him forgetting last year with cheating, and in addition, to you it's a symbol of his not making you a priority.
I don't think you are overreacting. I think this IS a big deal, for all of the reasons I've stated. I do like your H asking that you 'not make this a big set back.' Does he understand that we don't get to pick and choose what triggers us?
Is your H particularly busy right now? Are other things happening that would cause him to forget? It's right after Christmas, so it's not like there isn't a significant event that would be associated with a reminder of this day.
I'm sorry you're triggering. Not fun, at all.