I'm going to lie and tell her boyfriend we were flirting for a month.
But you weren't actually flirting with her boyfriend?
Nah, but I think about him a lot and want them to break up so I can have him.
It is wrong to try and break two people up. Wanting to date someone who is in a relationship does not give you the right to try to sabotage their current relationship.
I don't care if it's my right. I want to date him so I'm gonna do it.
Think about how much pain you will cause him by forcing him to go through a breakup. Secondly, if he ever finds out you lied about this and caused the breakup, he will want nothing to d with you. It would be best if you left their relationship alone and focused more on yourself.