De-lurking to say my bit 
Please think carefully about all the little niggly things that have led u to checking his phone, maybe write them down some where,
I had feelings like this about my dh 2 yrs ago, something just wasn't right, so I looked at his phone, found @friendly" messages to a female colleague that he rarely mentioned & calls too, found selfies of himself that he'd never sent me, but never anything concrete that I could "prove",
His behaviour changed, long work hours, tired or irritable at home,
The more I looked into it all, the more suspicious I got, the more suspicious I got, the more distant I got, etc etc
One day I just figured I was so lonely/unhappy sneaking around looking for concrete proof, I may as well just ask outright. If it caused a row & he left, I felt u would be no worse off..
So one morning, we took kids to school, and when we got back home & he was just about to leave I just caught him completely off guard by calmly asking "what's going on with #*%# ?"
He looked shocked, half laughed & asked what I meant,
I said I had no questions to answer, that he should start talking,
He broke down in tears & confessed all.
I was so angry when I realised that it had been going on for the same amount of time that I had been suspicious for,
And whilst my anger at him was massive, I was so angry, frustrated etc with myself that I hadn't trusted my gut,
I did know him after 13 yrs and my gut had been right, I just wasted time looking for evidence that was being hidden/deleted.
We are still together, & doing well,
Still have the odd wobble with myself, but he does everything he can now to show me he is a safe partner again,
He sobbed when he realised I had known all along, & that the more suspicious I got, the more I pushed away, so the more he turned to her..
Sorry, this turned into an epic post.!!
I wish you well,
Please trust your self, you know if things don't seem right,
XxxxX