I understand how you must feel. In the last 10 years I've lost both grandmothers, an aunt, an uncle and my dad all before their time. I found out about my uncle on my birthday of all days, which birthdays were already hard. I too had potential. I was smart and athletic, but I ruined it all. I also get the feeling of loneliness all too well. I'm almost 40 and I've been single my entire life. No one has ever shown genuine interest in me, not even when I had things going the right way. Hell just friends are hard enough for me to have. I ended up dropping out of school, then got fired from the only job I've ever had. You're looking at King Failure right here my friend. That dark place you mentioned, it's haunted me for 20+ years now and just won't let go.
So believe me you aren't alone in how you feel. It sucks, I know, so I truly hope you can dig yourself out because that can be really hard to do. I failed to do it, but maybe you can. I hope you can somehow find a way to enjoy your birthday, and even if it doesn't mean much you have my best wishes and I wish you luck.