Boyfriend has been flirting on Facebook, please help?

[Ruby]

I've dated my boyfriend for two years and for the past couple months he has been flirting with his ex, first harmless but now with more passion and yadda yadda yadda. I've confronted him about flirting with girls, and how it bothers me, but I never brought up this girl specifically. Mainly because, if he found out I was reading his Facebook messages, which is a total intrusion of his privacy, I would feel guilty for doing so. I don't want this to end badly, especially since I love him so so much. We both did wrong and have broken each others trust in that I read his private messages and he is flirting with this girl.

I can't stand dealing with this and not confronting him about it, what should I do? and how should I deal with this?