Sorry to hear that you had a lapse. I know it's a big deal to you. It doesn't have to be any bigger than it is, though.
I am not an alcoholic, recovering or otherwise. My BF is, and very communicative to me about what his alcoholism, and his AA participation, have meant to him. So, while this is second-hand advice in a way, from almost four years of pretty continuous dicussion about these issues, I offer it freely for you to take or leave.
First off: can you get yourself to a meeting in a place where you might not feel so overwhelmed? I think the general idea is that the drinking should be under control before other steps are taken, and that if there's a lapse you should address sobriety first.
Also first, actually: do you have a sponsor now? If not, can you figure out how to get one? This seems to be exactly the sort of situation designed for that one-to-one support. For one thing, to depend on your fiancee and your T for support is a pretty narrow circle. It's helpful to share the wealth, so to speak, so you can get support without feeling like you're exhausting everyone, including yourself.
Coop, I don't know how long you are going to keep punishing yourself for what happened to you. (Sorry to interpret--but that's how I see the drinking, in part.) It does not have to be endless, though. You can find another way. You can continue to take steps to live and think and feel differently. The course of your life is not pre-determined. You have the capacity for change.
Best of luck to you.
I'm just a poor wayfaring stranger, a million miles away from home.