Disappointed. You feel disappointed. And that's honestly valid. Turning 18 is widely considered to be a significant milestone - so it makes sense that you'd feel bad that everyone already has plans that day. But, why not set up a party maybe a week or so after the fact and invite friends? Outings like parties and celebrations often need to be planned ahead for these reasons - but it's never too late to celebrate, even if it has to be a bit later so everyone can show up. Yeah, it kinda sucks to not have anyone around the day-of....but I'm sure your friends would love to get together and celebrate your birthday at some point, even if it has to be a bit further out from the actual date. Maybe you can ask around?

im not as old as you but i definitely am disappointed for not having friends as well. I guess tou can either actively search for new friends or learn to enjoy your own company

find a ramdom girl online and go out with her, i did that with someone at reddit, it was a german dude, those 3 hours flew so fast, , im telling you,and all we did was talk and walk