NTA

Many people and cultures set aside the day of death for a very close loved one without an expiration date.

Maybe instead of calling it a dark day, rephrase it as a memorial day. One day where you give thanks for knowing a wonderful human being and you reflect on your memories of her.

How you label something conveys a different emotional response in others. If it didn't billions annually would not be spent on marketing.

I think your gf needs to have a different narrative than you mourning.l in morose darkness. Mourning also has a connotation that it's finite. She's wondering when you'll be "over it". However, "Annual Memorial" is a more positive spin and indicates it isn't going away.

If she can't wrap her head around a singular day that you need for yourself and is important to you, she may not be the one.

I hope you enjoy your annual memorial and find light when you reminisce on your happy memories of a lovely soul.