So your husband confronted you knowing something was very wrong, instead of using that as a catalyst to get to the bottom of your marital issues, and why this may have happened, you completely denied it, aside from to say you've been 'talking to a friend'. DH knowing you are full of shit but without proof and not wanting to call you a liar, then withdrew into himself and you thought.. 'well he's obviously not bothered, I'll keep disrespecting him and our marriage by carrying on this affair right under his nose.'
Said no loving spouse ever.
You've done this purely for the thrill it has given you, a little ego boost, which doesn't sound like it was ever going to be realised to be fair, you even kept his sexual messages so you could go back and re-read them, which is what led your husband to the evidence, it's pretty horrifying that you would do all that, deny it when asked, carry it on, LIE and BLAME your husband for why you did it, and then go snooping into his phone and destroying the evidence so he couldn't leave you.
I think you need to get off mumsnet and have a good long look at yourself before you put your husband through more 'therapy'.. I hope he is enjoying his holiday without you, it sounds like he needs the break.