I'm going to ditch my boyfriend on valentine's day
What did he do to ever deserve that?
Because he stabbed me in the back on this fucking day.
You shouldn't plan to get back on someone on a day meant to celebrate relationships. If you resent them for their past behavior then talk to them about this issue instead of trying to get even.
I'm going to tell him he doesn't matter to me anymore.
It sounds like you have a lot of relationship problems. Instead of trying to take revenge, why don't you try to make the day normal to make up for the both of your mistakes? Don't you want to have a happy relationship?