Husband cheated on me once, says ego rush from women's attention made him do it, need help

Two weeks ago my ADHD husband woke me up at 2 a.m. to tell me that he had cheated on me two days prior. He says that he was very lonely, and he had been feeling this way for a few months – he says he missed me and the way our relationship was before we had our baby (we have an 18-month-old daughter).

He said he didn't tell me because I'm so busy with the baby, and then with trying to spend some time with him after she goes to bed, and he knew I couldn't do anymore than I was doing already. So he started talking to people on some iPhone app, because he felt so lonely. He made plans to meet up with a woman at lunch to just chat. But I know my husband – the thrill he gets when women pay attention to him, especially if they come on to him in a sexual way, is more potent than I'm comfortable with, and I've known this about him for a long time. Attention generally, but sexual attention from women especially, really turns his head. I see it in his eyes – he is just energized by it. We have been together for 18 years, and he has never done more than flirt (non-sexually) and bask in the attention. I've always trusted him to know where the line is. Our relationship has always been very important to him. We both know we have a good thing.

So he meets this girl to chat, and he says that the flirting started to get sexual. And the adrenaline rush, the thrill of it, kicked in. He told her he was married. She said, "why don't we go somewhere more private?", and he agreed – he says that he didn't think anything would happen – it was a game of chicken, he says, the two of them testing each other. Again, I know my husband – he also likes competition, games, playing with risk. He got into her car, and honest to God, he says he had no idea what was going to happen next. Could anything else be more thrilling to an ADHD man (*Sigh*). I think I get it, but I also think he passed the "this is a stupid bad idea" line waaaaaay back. He tells her he's married again, and that they shouldn't be doing this. He told me that he expected *her* to stop it (?????). She says that it'd be better if they sat in the backseat. They do. She asks if she can kiss him, he says no. She starts to give him oral sex, and he says at some point, he told her couldn't do this, tucked Mr. Happy away, and got out of the car.

When he told me this, he started to cry, and said he knew he had put our relationship at risk, that he had done a really stupid thing, and he hoped I could forgive him. He said our family means so much to him, and it kills him that he might have thrown it away. He says he felt so lonely, and so low, and he needed to feel something. He said he knew it was a bad idea while doing it, but he stupidly felt like he was in control the whole time, and that he would never let anything happen. He says he should have stopped it sooner, and I told him he shouldn't have been flirting with strange women at all, virtually or in person. I told him that counselling for him is a must-do, no more fighting me about it (he has resisted seeing a therapist to deal with this ADHD for years). He also told me that he took an STD test, and will be taking them for the next three months, because the doctor he spoke to at the walk-in clinic told him that you actually can catch STDs from unprotected oral sex. And since he had sex with me the evening this happened, I might have to get tested too.

At this point, I feel alternately numb / resigned / furious. I don't know what this says about me, but while I am hurt that he didn't stop it sooner, or even talked to strange women at all, I am not exactly shocked that it happened. Does that even make sense? I feel like as soon as he told me he was meeting a strange woman for coffee whom he chatted with on an iPhone app, I already knew how the story would go. When he gets low, and lonely, and sad, he makes terrible decisions. He said that he usually leans on me to get him through it, but now that we have a family to take care of together, he realizes that he relies on me too much to get him out of his moods.

I feel that because I know him, I can see my way to forgiveness because in 18 years he's never messed up like this. Oh, he's hurt me in other ways, in worse ways maybe, but I could always count on his fidelity. On his realizing, eventually, that what we have built together is a good thing worth fighting for. But that's a certainty I don't have anymore – I feel like some part of him wants our relationship to explode, like he's testing the limits of what I'll put up with as proof of ... my love for him? his worthiness as a person? I just don't know.

I am troubled by the "rush" he says he needs to feel to make him feel better. I understand that after 18 years together, he's probably not going to get that same thrill and high from me, and while my younger self would have been deeply hurt by that, I now understand that long-term relationships have pros and cons like anything else. Familiarity and security are wonderful, but the cost (especially for someone with ADHD) are the brain cocktails they need to feel good – the thrill of risk, impulsivity, sexual newness. I am at a loss for how to deal with the anger I have, that he put our family on the line for a sexual thrill and ego boost – selfish desires. In my head, I scream at him 'why are you so weak? is the ego boost so important to you? what about us?' I know that someone else's mouth was on his penis and I want to vomit. He let it get that far, and I want to kick him and slap his face and humiliate him the way he's humiliated me. But this anger contradicts the faith I have that we can get through this, because he does seem genuinely remorseful, and because after 18 years – I know my husband. But what happens when he gets lonely and low again?  Will he have the willpower to avoid chatting with strangers, or will he just succumb because I didn't kick him out? 

I know this is long, but I need help. On top of this, I'm trying to take care of my baby daughter. I get through the day ok, and he and I are operating as normally as possible, but I feel like I'm just barely keeping it together. Any advice, please.