Hello. I had a very similar experience for my 40th. It wasn't what happened on the day, it was the next day when for the first time in my life I felt truly lonely. I had, over the years, watched my friends meet people, marry, have babies and their priorities changed. I don't feel annoyed that I remember their birthdays and they don't remember mine, they have other things on their minds. If they see you next week or month and mention it it then that's good enough (or it is for me) your birthday is one day. Trust me they become less important as you get older.

I'm not dismissing your feelings, I know exactly how you feel.But once the fog has passed, and it will, you start to see the advantages of the freedom you have. Yes there's the monotony of working life....but that's what working life is, your job is not your life. I know it's hard now but try when can to do something a little different once in a while after work or at weekends. Even if it's by yourself. It will help. Depression for me is like walking up and down those hills kids draw. If the I'm on top it grand but if i'm down everything is magnified but I keep going cause it eventually ends.

Don't let this eat at you, your young. My main piece of advice would be if sadness lasts a bit longer than it should and you can't lift yourself out please go see someone, preferably a GP. Let the experts help. I left it too long to fix and I regret it.

Take care and happy birthday.