I'm going to msging my boyfriends ex girlfriend
Why would you message his ex?
I want to talk to her about their past relationship.
Even if you are curious about your boyfriend's ex, I would discourage you from messaging her. Your boyfriend might get upset at you for going behind his back, and you might stir up old problems you don't even know about.
Fuck that, I'm going to message her right now then.
Why not just let the past stay in the past? It's not really any of your business to begin with.