Name changed as some friends know my regular handle.
Met DH at 18, together 14 years, married 9 with the young daughters. We both work full time, nice home, no money worries, perfect marriage on the outside.
Last year I found explicit texts between DH and OW, they were one sided- he was being a total creep and she was the one who alerted me because she thought he was a weirdo and was using WhatsApp with a picture of me and him as his profile picture 
we had a heart to heart and he was very apologetic- put it down to stress and wanting his ego stroked (it backfired massively). I told him it was a final warning.
Yesterday his phone pinged and his reaction pricked my spider senses. I logged onto his snapchat I know this is bad on my phone and he's messaging a random woman in Scotland (we live in SE England). I can't see the nature of the messages but the snapchat icons tell me they message a lot.
I've asked him to leave. DM and MIL think I'm over reacting as obviously he's not had sex with the Scottish woman on account she's 800 miles away... 
he's breached my trust, again, and acted like a 12 year old.
I'm in the right aren't I? 
I feel like my stomach has been knotted 100 times.