So are you saying that you only read one of the messages? The one you saw is awful and suggests that they have both been vilifying you. He has clearly painted you to her as someone who isn't worth "wasting energy on." What a horrible thing for you to have to read, I am so sorry. 
What you're also telling us is that this man used porn (until he got caught), signed up for a dating websites (until he got caught) and has now had an affair (until he got caught). So his response in recent times is to hide all his activities because he knows you've previously turned detective, checked his mobile and hence got a second one. His action in throwing the new mobile away is so that you couldn't see what else was on it.
This is a man who will only admit to what you can prove.
In the past, you have perhaps foolishly bargained away his behaviour by taking the blame for your lack of sex life. His response throughout, seems to have been "but you made me do it" and you have concurred.
So you ploughed efforts into restoring your relationship in the mistaken belief that this would prevent his porn use, dating site desires and infidelity. This is what I often call the "prevention myth" because as you have found, you could have been a cross between a porn star and mother earth and yet nothing can stop someone being unfaithful if that's what they want to do.
I sincerely hope you don't bargain this away again or accept any blame or fault for what have been his behaviour choices. It might interest you to know that later infidelity often starts with porn use and given the rubbish he told you about the dating site registration, I would think this is as simple as he has been looking for someone to act out the porn fantasies he has been engaging in for a long time. Unfortunately, he has clearly met a woman who was idiot enough to believe all his tales of woe about a neglectful wife, so much so that she has dehumanised you, as evidenced by the words in that text.
He is being no more honest with you now than he has been before, when he's been found out. I would hate being lied to, so I probably would go and find that phone, but I suspect he will have got to it first and in any case, he will just buy another one.