Hi Ringlet! Welcome! There's a lot of caring and support here-glad you joined us. I don't think alcoholism and opiate addiction are all that different, not from what I've learned. Addiction is addiction, pure and simple-the mechanism is the same, it's just (as someone told me, can't recall whom) "the method of delivery is different"-drink alcohol vs. pop (or snort) a pill.
I broke up with my ex Friday and the next 3 days, I spent trying to make sense of his behavior. Now i've moved on from that. Maybe it's like, moving into the Kubler-Ross stages of grief (if you're familiar with that concept).
Now I've reached acceptance of his condition, and acceptance that we can't be together. I love him, and it hurts like the devil to be without him, but I'm steadfast in my determination to craft a better life for myself.
I'm so sorry that he left you like this--so suddenly, wiht very little explanation. I know you must be in tremendous pain. Maybe he had already started using again and didn't want you to know, so he left.
Unfortunately, I don't think anyone here can tell you exactly what was going through his head, I wish they could.. but maybe people can share their similar experiences, and you can gain comfort from that.