Wow, so much said in that small paragraph. Well I'm glad to see that you kicked your drug addiction, and sorry to see that she fell into one. I absolutely see your point about not believing what she is saying about her relationship with the girl she is with all of the time. I too think that after having a failed sexual encounter with a friend that things would be too weird to continue the friendship. Chances are she's still sleeping with her. She probably told you because she thought that you had figured out what was really going on. She probably figures she'd tell a half truth so that she looks only partially guilty, and then hope that you'd forgive her and move on (which it looks like you did). The random, frequent, and seemingly loveless sex (you said she usually leaves immediately after sex) is an unfortunate sign of a guilty cheater. Some cheaters try to compensate for their treachery by buying things for their partner, while others will try to f*ck them much more often. Both are attempts to make the cheater feel better about what they are doing to their partner. They feel that if they do more of what you want (or have sex more with their partner than the person they are cheating with) that it somehow makes things right/equal or makes them less wrong. Its a twisted way of thinking. Her sex with you has become payment/restitution in her eyes, and when she feels like she has "paid" you or "done her duty" she leaves. That is also why she gets mad if you say "no". She isn't able to unburden her guilt.
As far as your feelings and not wanting to end your relationship simply because you've been through so much.... If the love isn't there anymore it just isn't there. Sure she may have stuck by you when you had an addiction, but you didn't cheat. Had her situation been only about the drugs I'd suggest staying by her side because there was still love there and because she did the same for you, but there is another person involved here. No matter what went on in the past or how good things used to be you have to look at the present and the future. If someone else is coming between you two, you are losing desire for her, she is obviously still cheating (emotionally and/or physically), and she is still doing drugs what does the future of your relationship look like? Bleak at best. If she can cut this woman off, and start trying to get clean then you two have a chance. If she doesn't want to do BOTH then you need to move on and let her shoot heroine while her friend munches her pu*sy. (sorry in advance if any female members are offended by that word)
...one can be sure of nothing until it has already happened...