I’m so sorry that you’re here, Jabell73. Sadly, your marriage hasn’t been real from the beginning. There has been a 3rd party, unbeknownst to you, in your life and your relationship and yes, in your bed—because he surely exposed you to everything that she was exposed to from however many men she was with. He has literally put your life in danger along with your emotional and mental health.
You are probably still trying to absorb and process the reality of your life and marriage and spouse, so it’s pretty normal to reach for explanations of this inexplicable and horrific betrayal.
I too question the sex addict label, particularly here. Is there even such a thing as a sex addict who’s only addicted to sex with one 18 1/2 year affair partner? I don’t think so. He was possibly addicted to the ego boost, the secrecy and sneaking, or maybe he just didn’t want to stop having sex with her, but that isn’t sex addiction. Gently, you may both be using that label to partially excuse his behavior. I’d be very very skeptical of this and also skeptical of a therapist who casually agrees with that "diagnosis" unless she/he is certified specifically to treat and diagnose sex addiction and has taken him through all of the diagnostics for it. You don’t diagnose sex-addiction from a conversation.
Also gently, you say that he’s so remorseful and has been doing everything possible to make everything right. How on earth do you make complete dishonesty and betrayal from the first day of your relationship right??? You had to discover it for it to stop. He didn’t feel bad about it and stop. He didn’t confess it to you after ending it. You have not had a faithful, loyal, honest spouse for a single day of your life. I hope that you are right about his remorse, but usually when a WS is caught after such a long-term betrayal, they are regretful that everything has blown up in their faces, but remorse is much harder to come by. What is he doing specifically to make everything right?
You are right that this has changed your life forever. The worst is that it was always changed, you just didn’t know it. It’s truly a very hard pill to swallow to discover such a long-term betrayal. In my case, I’ve come to believe that my life was never what I thought it was. . .and I really loved that life/marriage that wasn’t. The loss is so hard to live with.
Please take care of yourself. Get tested for STDs if you haven’t already. Be gentle and patient with yourself. And if you’re only in marital counseling, strongly consider individual counseling just for yourself. Right now, this isn’t a joint experience. What you’re going through is singularly your experience rather than a "we" experience. You need somewhere and someone to help you process your own feelings and experience through all of this.
Sending you hugs of support.