Oly, I'm so sorry you're going through this. Stay strong.
Re: "It is always going to stay unattainable, and therefore sexy." -- this is sometimes really the only advantage the "outside" partner has over the partner on the homefront. I say this as a former outside partner. But the thing is, unattainability is not an actual trait of R's -- it's not part of her character, it's situational. Whereas the wonderful things that made C love you are part of your character, you developed them, they will always be with you.
If she ever "sobers up", she will see that.
I empathize with the sexual safety thing too. I was once married to a really outrageous cheater (I got so tired, eventually, of all the people ratting him out to me, about so many different women!). But I also had a really awesome family doctor. He was a quiet, older man, the kind of doctor who you'd think would avoid talking about anything sexual with his younger female patients. But after i asked him for a round of STD tests once, he asked a bunch of probing questions and eventually became the one person in my life who pushed me, relentlessly, to get out of my marriage. There was physical abuse, too, and he was the only person who knew about it because he had to treat me for an injury once. But even before that injury, he was relentless in grilling me "are you using condoms with your husband? Are you? Every single time?"
Sometimes I think that the safe-sex stand I took with my ex husband -- because I didn't want to let down that doctor, or have to tell him that I wasn't protecting myself -- was the only real stand I ever took in my crappy marriage. Seems pathetic, now. But it made me feel a little bit strong. So, insisting on protecting yourself in your situation may be just as important for your mental health as it is for your physical health.
Leelee, all alone.