A lot of your insecurities are stemming from his bad behavior. You've seen him look elsewhere for sexual gratification and despite you telling him you were uncomfortable with what he does on Craigslist, he persisted. You set the boundary and he crossed it anyway in secret.

Him telling you that he'd want to be with someone else if he wasn't with you is awful and would definitely make me self-conscious, too. This guy is not good for you and you're not irrational for feeling the way you do, so you don't need to feel guilty for having these feelings. You should be with someone that builds up your self-esteem--not someone who constantly makes you feel like you are not enough. I know that you like him and you seem to like your relationship overall (95% of the time at least), but this guy not only secretly goes behind your back to get his kicks from strangers, but disrespects you in front of your face by saying he wishes he could have sex with another person.

Sure why would he stay for so long just to cheat, he says. I pay for everything. I'm always there for emotional support.

That explanation is not good for explaining why he wouldn't cheat! Maybe "I love you and I'd never cheat." "You're enough and I'm not looking for anything outside of this relationship." Length of a relationship is not a good reason and it's not something you'd say to soothe someone's fears of cheating. Also, you sound resentful for paying for everything and for providing him with emotional support based off of how your phrased it. This doesn't sound like a fair power balance--especially if he's not giving you emotional support back.