Help! I got into my boyfriends Facebook account and...?

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My Husband and I have been together for 10 years. About 6 months ago he left his Facebook on our game console TV sign in and I recorded the password because he has proven to me he is a liar as well as saying in anger he should just cheat on me . I logged into his account a few times and noticed he was flirting and sending messages with other girls but none of them responded back so I kept my mouth shut about it. Now today I just happen to log into his account because he left all day and never called or text me so I started getting suspicious. There is another girl from his past (HS days) who gave him her phone number and told him to text her. I am quite upset and pissed off all at the same time. I know this is not the first time he has attempted to cheat on me but it is more upsetting because now this girl is reciprocating it. I don't know how to go about this I guess because I have "snooped" in his personal account so that is why in the past I have let it go. I am just really hurt and feel betrayed as hell right now. Things were going great for us too recently so I am so confused why he would do this of all times. When we are having problems yes but when we aren't why would he want to do this to me? I don't need to hear I was wrong for snooping, I get it. I just want advice as to how I should approach and confront him on this. Thanks
Something Fishy- Maybe you need to re-read what I wrote. How did he show he could be trusted? Lol. Yeah, if that is trustworthy behavior than I'm the Pope.