It's curious that I find myself playing this at my bday's 00:30 hours (I haven't even slept yet), but I'm glad that I did.
First of all, I agree with all the previous comments: the pacing, the transitions, the startling "happy birthday" sound effects, the writing; everything you've poured into this bitsy works perfectly in tandem to convey tension and unease. In these very minimalistic elements, you've been able to condense such deep thoughts and feelings.
Secondly, it takes a lot of courage and self-honesty to open yourself up to strangers, even through a medium like this. And in doing so, I think that you've found that you're not alone in those feelings. Personally, I cannot say that it's my bday the specific date that triggers my sadness and anxiety, but rather I constantly feel day to day.
And in creating this game, you've not only distracted yourself, but you took an upsetting feeling and turned it into a beacon to shed light onto yourself and others. So I strongly applaud you and I thank you so much for this! Keep on creating!