I'm not sure you can forgive him until he is really sorry. And at the moment he doesn't seem to be.
Not intending it to go any further is one thing - but is that because he didn't want too, or because she was too far away? And why does he think that sex texting and phone calls are okay, but taking it further wouldn't be? Would he allow you to exchange sexy texts and pictures with other men?
He needs to realise that what he has done is wrong, and has really hurt you. And he needs to be genuinely sorry that he's hurt you in this way, and not just sorry that he was caught. Then you can try to rebuild everything, and fix the problems in your marriage.
Relate could be a good idea, as long as he is really sorry.
I think I'd tell him that - that you aren't convinced that he is really sorry, that you aren't 100% sure that it wouldn't have gone further had she lived next door rather than so far away, and that you are very hurt that when you suspected something was wrong and could have tried to fix this earlier, he attacked you and blamed you rather than coming clean or even just stopping the texts.
His response will say everything. He'll either try and win you back, convince you that he made a mistake but he loves you and he's worth fighting for, or he'll decide it's too much like hard work. Either way, at least you'll know where you stand.
As for the thoughts - they'll ease once your mind has processed things, and they start to get put right. If they don't ease, something isn't right yet. Think of them as a defence and survival mechanism.
Good luck - this isn't a nice place to be.