Background
I turned 23 yesterday. Last year, my mother forgot my birthday, mostly because she was working in a country in a timezone 6+ hours behind mine and didn't realize it was that late at my place. I was in a really dark place at that time, which mostly explains why I got mad at her for forgetting. I called her in tears and told her she forgot me on an important day, and she profusely apologized for not wishing me my birthday. Couple of times this year, she mentioned the story during family gatherings and told my relatives how ashamed she was for forgetting.
Current situation
I'm no longer in this dark place and she knows I've healed and am happy now. I was waiting for her call yesterday but it didn't happen. I didn't want her to realize day(s) later that she forgot again and felt bad about it so, at 9:30 PM I jokingly texted her:
So, it seems like you forgot which day it is so I'm texting you in order to avoid the crisis we went through last year ^^
To which she responded by immediately calling me, and I could feel she was on the verge of crying. She just left work (which explains why she didn't think about it) and felt terrible for forgetting again, profusely apologized and insinuated throughout the whole call that she is a terrible mother and that I don't deserve her. I felt terrible because the whole point was to avoid her to feel bad. She wasn't used to forgetting my birthday, it's only the second time. I used to make of birthdays (mine, and others') a pretty big deal; I don't anymore, but I don't think she knows about that.
Later on that night she continuously texted me to apologize again, telling me how much she loves me and that I don't deserve her.
What I tried
- Telling her (both on the phone and by texts) that it was okay, that it was not a big deal and that I loved her. It didn't stop her from sending me more of those messages and I'm afraid she'll try to make up for it by buying me an expensive present that I don't want both because of my own guilt for making her feel that way and because I don't need anything.
Question
I would like my mother to forgive herself for forgetting my birthday and let her know that I don't hold any grudge at her because of it. How to reassure my mother about the fact that forgetting my birthday does not mean I'm mad at her?