Thanks Catwalker - this is a long, difficult journey ahead, but if you love each other enough, it will be worth it.
I went through a similar experience and discovered a one-off affair (two assignations though) 18 months ago, after 24 years of marriage. Mine was also an accidental discovery via text messages on an old phone.
You might be going to counselling too early, or you might have found a bad counsellor. If your H is telling you - and this accords with your view - that his affair had nothing to do with your marriage or his feelings for you, seeing a counsellor who is insistent that these things cannot happen if a marriage is happy is going to be counter-productive and make you feel worse.
Affairs like your husband's are all about him. After 18 years of what has probably been a perfectly happy marriage, along came someone who reminded him of what it felt like to be desired intensely. I would imagine your husband became addicted to the feelings this affair induced - and not the affair partner. It is really important for you both to understand this - and will explain to you why he was able to give her up relatively easily and also why he continued to text her afterwards.
I completely understand why he wanted to normalise things. Having realised with horror what he had just done, he would have been at pains to try to end this relationship with the least amount of negative feeling. This was rather naive of him, but I can understand it. However, there was probably also a part of him that was addicted to the buzz of being chased and so he needs to think about what he was also getting from the continued contact.
I also imagine he believed that there was no point hurting you with an admission about something that in the great scheme of things, meant nothing to him. This is completely flawed thinking, because secrets like this always damage intimacy in relationships and therefore yet again, I can understand his relief that this is now out in the open.
That you know about this is a great gift - your mantra together now needs to be no more secrets.
The path we took was by talking and talking about what had happened. However, my H was in a bit of denial about his motivations and so after 2 months, booked himself in for some counselling. This was really productive for him and made him examine his character in greater depth. Throughout all these months, we have reviewed our entire relationship and my H has made the most enormous changes to his character and behaviour.
It was an opportunity to get the marriage and the husband I always wanted and our marriage is infinitely more rewarding for all that work.
Does he still work with the OW? That's going to be difficult for you, as there are 18 months of contact that you will need to know about. Continuing to work with an affair partner is always a mistake.
I would really recommend a book called Not Just Friends by Dr. Shirley Glass. This will help you make sense of this affair and it will certainly help you when she explains the "prevention myth" - that is that there was nothing you could have done to prevent your H's infidelity. Only he could have prevented it.
Some of the difficult questions that might be going through your mind (and will need answering) are - was this the first time in your marriage that he was presented with a cast-iron opportunity? If yes, this will lead to some uncomfortable self-analysis on his part about whether he would have said "yes" at any other time too. You might both need to face an uncomfotable truth that the fidelity in your marriage was preserved only because of the absence of opportunity, as he might not have ever been the sort of person who would have courted an affair.
In the coming weeks, you need to talk about your attitudes to temptation, secrets and infidelity. You will also need to examine who has been the over-benefitted one in the relationship. One of the common myths about affairs is that a partner strays when they weren't "getting enough" when it nearly always emerges that actually they weren't "giving enough" and that their needs were being met pretty well.
I also want you to do the maths. and weigh up how your husband has behaved towards you in the past 18 years. If you can see that this awful event and its aftermath was an isolated blip in someone who has on the whole been loving, kind and nurturing of you and your relationship, it is easier to have hope for the future.
Your H needs to understand above all that there can be no "going back to normal". You will feel hurt and betrayed for a long time to come and he really cannot talk enough about what this meant and why. There can be no brushing this under the carpet and "moving on" for a long time. You will see various timescales, but in situations like this, it can take up to 2-3 years for the betrayed person to feel a sense of order in their world again.
The good news is that there are lots of us on these boards who can really help you. What ever you are feeling, I guarantee one of us (or even all of us) has felt too. Do keep posting - it really helps to write down what you are feeling.