Thank you for your responses. I feel like I really need a more objective point of view; sometimes hearing this stuff from people who don't know you (or your partner) well can be helpful.
Anneintherain, he did break a safer-sex agreement. This is not the first time he's done so, and not the first time we've had an STI scare. But the first time, he kept saying how much he had learned from it, and how things would be different in the future. Obviously not.
SchrodingersCat, I'm pretty sure that it doesn't have anything to do with addiction, he's not really the type (then again, I have been wrong about a lot, apparently...)
I think he resents our relationship sometimes, and doesn't necessarily like the safer-sex boundaries we have agreed upon (but they were in place precisely because of situations like this...a partner NOT being informed about their own health, etc.) As I've said to him before, he is free to do as he pleases but I will need to change my behavior accordingly if he does. The fact that he was physically intimate with me after receiving unprotected oral sex from someone with HSV was one of the worst parts of it for me, worse even than being raped (when that happened, I had no reason to trust the person who did it. I trusted my partner and he violated my consent and possibly put my health at risk.)
It is complicated by the fact that when he got involved with this new person, we were having some difficulties in our relationship, and he pretty much just blew that off to spend time with her.
I am just having a very hard time with all of this. I have loved him so much and been so open and vulnerable with him and all of a sudden it's like I just don't know who he is anymore.