Why do you keep screwing up!! Stop talking!!

You say "that's my birthday." If she asks why you said you just had it you say "honestly I didn't realize it was so long ago. I've been really busy with <work/passions/taking care of...> so I've been out of it lately. You've now established you have a life and shown vulnerability as well as a willingness to face up to mistakes. If you are actually busy you've let her know so if you aren't always readily available she knows it's not disinterest; if you aren't actually busy you just said you were. Don't be a liar. Now you HAVE to start pursuing passions or career advancement or else you'll look like a sack. Even if y'all break up you've now set yourself on a path of potential success and self betterment. Your ambition and achievement will make you a more desirable partner and you'll start attracting more matches. You're welcome.