Is my high-school sweetheart having an affair again, or am I paranoid?

We met when I was 15 and he was 17. I feel like it's important to say I'm a Scorpio and he's a Taurus. We're both very equal/opposite, intense, headstrong, and we seemed to fall in love very quickly. I'm now 21 and he's 24, we are married, and own a home.
He has cheated once about 3-4 years back. It was about 6 months before our wedding when he was talking with one of his ex-friend's younger sister. It was all over texts and phone calls. He says he initiated it and they talked about their day, and sexual things like what they want to do to each other. As far as I know, there were never any pictures or anything and never met in person.
When I found the messages and confronted him we both broke down. At this point it is relavent to say that we were both incestually sexually molested as children, both in very different terms, and both reacted differently (he being more wild and myself conservative). He said this had to do with why he cheated. At that time, I had not yet told him I was also molested, and there were certain things I couldn't bring myself to do in bed. Because I wouldn't preform oral sex he cheated as an outlet.
The affair ended and we have since come a long we. We have talked a lot about our abuse. I am now much more open sexually. He sad settled down into the role of being a husband. However, I still find myself wondering if he is cheating again.
Gradually over the years his sex drive has faded. We have sex 1-4 times a week, but it's usually just one maybe twice. He occasionally initiates but it's usually me. He mostly just flirts but seems too lazy to get around to having sex unless I initiate. I've brought it up and he claims he is tired a lot from work, but that he is happy with our sex life. I asked if there was anything new he wanted to try or anything he wanted to talk about, but he insisted everything is amazing.
The second thing which makes me wonder if he is cheating most is on his phone. A few months back I saw an email to him from some company where you paid to hook up, and also a sent email to a Kelsi saying "can u send a picture?". He claims that he has no idea about them. He says the first must be spam (which I admit I do honestly get spams like that) and he doesn't know about the sent email.
I have since been keeping an eye on his phone. I know it's wrong, but I need to know. Months have passed since the email incident. I have always had all his accounts and passwords that I know of and they are always clean. But what if he has accounts I don't know about???
I recently saw a porn video he downloaded. No problem. I hopped on the site it was downloaded from BC he is shy about this stuff and doesn't like to talk about porn or masterbating. I just wanted to see what he was turned on by in case there was something new, but I quickly discovered you must have an account to download this particular video. So even more curious about what he was into, I tried to log in. None of his usual emails/passwords worked.
Then, a few weeks later, I saw internet history where he logged on to his regular email. Then there were several more sign ins (as if he maybe forgot the password to a different email.) He must have finally got in BC it showed an email saying ISO Daddy Dom for Little Girl. It then went to his deleted folder.
I could see all this on his history, but I didn't think of clicking it to see if he was using a secret email until it was too late. I went back another day, thinking I could calm my mind by seeing what email he was using, but that particular history was deleted.
What do I do? I can't confront him because I don't actually have any proof. Am I over thinking it? Even if he is cheating, I don't think he'd admit it this time unless I had proof. I've tried guessing a secret email but nothing works. Does that mean I'm wrong or that he's getting smarter about cheating?