Most Helpful Guy
If he says that you are the only one he wants and that no one will come between you. AND you do NOT say, are you gonna leave me for someone else, or something of that matter. Then it is because he knows he does talk to other girls and will leave you for another girl, and because he knows this, he has a subconscious mind thinking that you might know it as well, so he is going to try and reassure you he isn't like that by feeding you lies. Now if you do ask him if there are other girls, then well he has a solid reason to say those things. However, he does not have a solid reason as to why he's flirting with other girls. Well he does have a solid reason, but that reason isn't a good one. And the solid reason would be because he's unfaithful. I would leave his ass if I was you, and I wouldn't even explain to him why you are leaving him. He never explained all these other girls. So why should you have to explain to him. And because he has these other girls, he's going to act like he cares and will say anything to get you back, but really he has back ups.
Now this is just how I see it. I didn't read what he wrote, I am just going off of what you are telling me and this is my conclusion to it. Do what you feel you should do. If you want to talk to him about it first then talk to him about it, but just know, regardless if he is telling the truth or not. he is going to say anything he can to turn it around on you. Probably say you are probably have been cheating on him, and because you were getting away with it, you felt like he might be getting away with it so you snooped into his business. This will cause the argument to be focused on you and not him. Where you will be saying, I never cheated on you. What ever you do, don't let him turn it around on you. Be firm, and if he tries to turn it around, Just say you're done with him.
But at the same time, he could be very reasonable, and actually have good explanation (don't know how good an explanation has to be in this case but it better be pretty damn good) He might admit that he was wrong. You can forgive him, but I wouldn't forget. Because now, chances are, he will still do it (even though he will say he stopped) but now that he knows you know, he will become better at hiding it. Like erasing old messages and such.
Just be careful with this. Do what you feel is the right thing to do. Want to stay with him because you love him then do your best. If you want to just call it quits now, then do so. But know whatever decision you make, you need to stand by it. And don't be so quick to take him back with full trust. He lost that trust, now he has to earn it back. Speaking of which, he probably will lose trust in you if you tell him because he will bitch and complain why you are going through his stuff, regardless if it was an accident. More than likely he won't see it as an accident..
Sorry I'm rambling. Best of luck to you.