Ok, so let me preface this by saying that nothing can excuse the way he treated you or behaved, and I can't begin to imagine how you must of felt going through that. It sounds like an absolute nightmare.

That said.. he may not have broken up with you for ***her***.  He very likely broke up with you for the ***drugs***. And also not because he felt like you didn't deserve him, but because he felt like ***he didn't deserve you***.  

It depends what kind of drugs they were doing of course, but the way you describe it, it doesn't sound like he fell for her or fell out of love with you.  It sounds like he slowly became addicted to the drugs, then started spending more time there and less with you because of it, and eventually started doing more and more depraved things because of it AND for it.  

I mean.. for all you know this shit woman could have done this on purpose, slowly entrapping him until he was under the drugs' control, and used them as leverage to get him to sleep with her.  And then he just felt so ashamed and guilty for what he'd done that he couldn't face you anymore.  

Ever see a totally sketched out homeless person or prostitute and wonder how they wound up there?  It doesn't happen overnight. It starts with a little taste, and then it's a slippery slope all the way to the bottom.

Not saying you should forgive him for what he did or how he treated you. Or that you should want to take him back at all.  But if true, this might give you some context and understanding as to why and how something like this could happen.