Why Your Friends Shouldn't Give a Crap About Your Birthday
First of all, I can understand why the break-up was traumatizing, and it has nothing to do with the fact that it happened on your birthday. When you split with someone you'd planned a future with, you're not just saying goodbye to him -- you're saying goodbye to an entire imagined future. You lost him and the life you'd planned for. And your sense of who you were, if you'd built your identity (in part) around being his future wife.
Every relationship is unique and some people truly do know, that young, what they want. (Read this article and I'll spare you a long digression on the topic.) But you started dating this guy when you were 18. At that age you're still defining yourself, and if you define yourself and your goals and your future plans all by your relationship to another person, it can be devastating when that person leaves your life. The point is, at whatever age you choose to do it, know who you are before you start building a life with someone. You will grow and shape each other as you go along -- but you'll have an idea of what kind of person you want to become at his side. You won't simply be an accessory to his dreams.
So the first step is to separate your grief over the end of that relationship (and the loss of that particular future) from your birthday. They had nothing to do with each other. You say you knew things were headed downhill and his birthday slip-up was just the final straw -- so don't view the break-up as "That awful thing that happened on my birthday," but as "That awful thing I saw coming from miles away." Think about it: the events that put a damper on your special day were the product of extremely specific circumstances. You will not have another birthday like that, ever. You're not turning 21 again, you're not dating that guy again, you're not going to have that conversation with him again. So stop dwelling on it.
If that day is still haunting you, three years later (which is as long ago as you'd ever been together) it's not just the day that's haunting you, it's the boy.
Jane Austen wrote that "friendship is the finest balm for the pangs of despised love." Once you've got your girls rallied around you, though, nothing demolishes the last of your sorrow quite like moving on to a new guy.
You're young and active and happy and prospering, so bounce back already. How much have you dated in the last three years? I mean really thrown yourself into it and given people a chance and sought out new connections and made yourself available to new guys? Why not start going on dates with anyone who asks? A date is not a marriage proposal, so don't overthink it. And there are no bad first dates; just hilarious bad-first-date stories. Your last major relationship was in college, when most people our age didn't really "date," they just slid from friendship to cuddling to being totally inseparable. You need to practice dating as an adult. It's awkward. You'll get over it.
The second thing you need to get over is your birthday.