Happy Birthday Kate! I feel a lot like how you're feeling right now - i turn 20 tomorrow and it absolutely sucks for me too. I want you to know that you are not alone when it comes to feeling this way. Age really is just a number because emotions don't look at how old you are, you just happen to feel a certain way =\
Here's partly why I can relate:
-3 years ago was probably the last "Fun" birthday I had when I met my mum/grandparents and friends and went out of my boarding school for a party!
-2 Years ago I left for college and my mum and dad flew down to where I'm studying to wish me, my girlfriend and I went for dinner
-1 Year ago I was with my girlfriend, no dinner, just a chocolate mousse at midnight in a fancy hotel, I dropped her home 12:30 and took a long walk listening to all my favorite sad/thoughful songs.
Let's see what happens this year. In any case, even if it doesn't seem all that bad - what sucks is that I've felt low around my birthday time too (at least these past few years) because I feel like I'm just getting older and years are passing and I'm stuck, stationary and not doing anything worthwhile with my life - it's the worst thing and i feel this way a lot: I feel irrelevant.
I also genuinely feel like I wasted my childhood away and there's no way I will be happy on my birthday (I miss whatever fun and excitement I used to have many years ago - and it was from the little things like the new toys I would get)
So yes, I want you to know that you're not alone, go for a walk or take a plane to somewhere new, buy someone a drink (that's how I made my first set of "friends" in Belgium when i went on a solo trip)
Happy Birthday! I'll be sure to check on you next year!