It’s hard to understand the full context of your situation because there’s a lot of details missing, so I’m going to make some assumptions based on what you’ve said.
I think your mistake was to go straight to running when you haven’t even started crawling.
You’ve described yourself as not having been a very sociable person over the years, so planning a massive party of 30-50 people for people that you haven’t had regular contact with was likely to never have worked out the way you expected, regardless of how much effort you put into planning and setup.
My advice to you is to start small and take it in steps.
Firstly, don’t bog yourself down with thinking about how a) badly the party seemed to have gone, and b) how many relationships you’ve let erode.
Second, instead of focusing on those that didn’t show up, celebrate that you had 5 people who cared enough to come to it. Spend time cultivating relationships with them, because those were the ones who bothered to be there.
Lastly, for those you felt were important and let you down, you have to understand that people tend to treat big parties as optional. If someone is important to you, inviting them to a large party where they’re just one-of-many doesn’t really tell them how important they are to you does it? I know I’d feel a lot more important if someone invited me to their small and intimate party!
Additionally, you should reach out to them and let them know how you felt - no one can read your mind. You have to communicate how you feel and give people a chance to respond. If they respond positively, great - you’ve kept an important friend! If they don’t, then you’ve learned that your relationship with them wasn’t a healthy one.
You don’t have to pretend like it didn’t hurt you, but the onus is on you to communicate that to those you felt slighted by.
Also, don’t plan massive parties for yourself with over-the-moon expectations, especially not for your first birthday party!
You’re welcome and I hope things work out well for you!
Relationships are hard, both platonic and romantic. Maintaining them in a healthy way is really difficult and no one can give you a map for how to navigate the issues.
Sometimes, you’ll find that despite your best efforts, relationships can wane or end over time. Your best friend might suddenly have to move halfway across the world for a job - while they’d still be your best friend, you won’t be able to get coffee/beer like you used to. Or a close friend could unfortunately lose their life at the drop of a hat, and you’d never see them again.
I’ve learned to focus more on and appreciate the time that you do have with a person, however long or brief it may be, because you never know when it’s the last time that you spend the most time with them.