What do you think of me breaking up with her on her 21st birthday?
Do you think her birthday was a day that you shouldn't break up with her?
That was not one of them.
It's okay to break up with someone, but doing so on their birthday feels immature and spiteful. You should avoid doing something so vindictive in future relationships and treat former partner's respectfully.
Do you think that she deserved to have me be nice like all the times before this. It's her fault that I choose to do it on her birthday.
Usually people refrain from breaking up on someone's birthday. Don't you want to spare them from such a painful memory?