Today is my birthday and I'm depressed

Today is my (M) birthday. I just turned 39. And for the first time in my life, I'm feeling depressed on my birthday.

I have so much to be grateful for. I have my health, i own a home, I have a business where I'm my own boss and all my friends and family around me are in good health. I even had a handful of people message, call or facetime me to wish me happy birthday. I feel grateful.

Despite all that, I'm severely feeling the birthday blues.

I've come to the realization that my personal successes have always been a result of never feeling like I'm enough. Anything I've achieved in my life has been as a result of feeling wronged by someone or me bouncing back from a hardship in my life. As a result, I've done everything on my own. My house, I bought on my own. My business, I run on my own. I've travelled throughout much of the world, on my own. I do for others but never ask for myself. I've just become this "lone wolf" or rather, I've just built this fortress around myself to shield my life from vulnerability. I don't date because I'm not in the same shape I used to be so I feel like I can't attract a female , though I have no problem talking to them. I hold myself back in my business because I'm always afraid of the what-ifs. I'm always just awaiting the perfect time to take that next step in my life to finally reach my potential, take that risk etc. Comfort and familiarity have become my friends.

Maybe it's just because I'm about to turn 40 in a year that's got me feeling this way. I'm usually very happy on my birthdays. In high school, I used to have the same birthday as the most popular girl and people used to come up to me and say "Hey, did you know its (her) birthday today?" So I always made it a point to celebrate my birthday once I got older. But today, I just want to be left alone. I don't want to see anyone.

Can anyone relate?