>Can someone tell me what his thinking process might be?

I doubt it, but the one thing you didn't mention in all this is whether you've talked to him about it. If so, what did he say?

I'd think most of us would find this unfulfilling, but I also know that by now, I would have addressed it. In one sense, 2 years is a pretty long time, but in a larger sense, it's still part of the honeymoon phase. This reads like 2 people in their 60s!

>I’m not saying that justifies me cheating, but it may be the reason I did it.

If you want to save the relationship, don't say things like this. It's not even the 1st time you cheated. I'm not being judgy, just laying out a fact.

On the rest, this doesn't sound like a healthy relationship to begin with, so I'm not sure why you want to save it so badly. But if you do, you need to realize a couple things. Most of all, if he learns to trust you, this takes time. Giving him your passwords doesn't help him trust you, because trust comes from inside. Also, be aware that it's not "just" the cheating. You also proved to him you can lie convincingly. He's the one who has to work through this, meaning your job is to give him the space and stay patient.

Also, just a random observation. When you first got together, you were 19 and he was 24. This is a huge age gap, esp for you, and it might be part of this.