Wanted: Pain survivors and those with addiction experience

Greetings to the community. I'm looking for advice on a situation.

Before I met my wife, she was a heroin user. Based on her history and behavior of use, neither of us really categorized her as an "addict," but she was a user. She kicked it right before we met and stayed off it for years, promising it would never again be an issue (which I trusted).

However... She recently relapsed.

Owing to a number of factors, chief among them surviving cancer and (likely, though as yet un-diagnosed) RA, along with a number of other influences like family history and (probably) poor diet and exercise habits, she is in a great deal of chronic pain. We have spent years trying a great deal of medical (professional and otherwise) treatments to no avail. The pain was affecting everything; her mood, her ability to be productive, her ability to concentrate and achieve her goals, everything.

So, without my knowledge and (as was claimed) to her own shame, she started using again. Small but regular quantities to (as was claimed) manage the pain but not "get high." When I found out what was happening, I confronted her about it and insisted I be allowed to help rather than kept in the dark. She admitted I'd handled the situation in the most supportive way she'd imagined and agreed to cease use and seek treatment, attending a Methadone clinic within the next few days.

She has been a model patient; attending daily and regularly, passing all UAs designed to find usage of substances not prescribed, and completing her assigned therapy appointments. (Though, she does not take her take-home doses as prescribed, preferring to mete them out differently to deal with the pain in a more targeted way.)

However, despite constant dosage increases, she has not reached what she considers to be a "therapeutic dose" (described, by her, as "enough to remove the pain and not be jonesing for another fix").

This is all backstory. Where I need guidance is in how to deal with the current situation: She has become mean. She is grossly intolerant of most things, responds harshly and with malice to the needs of others, and has a generally sour disposition. She can't stand criticism, is unable to complete most tasks that require focus, and has lost all compassion within her. She sleeps most of the day (upwards of 15-18 hours), is incapable of bringing herself to complete chores (even the most basic, like washing a dish or changing a diaper on our child), and appears to have no interest in anything.

I have dealt with depression my whole life and I recognize and empathize with many of these struggles. However, I try to believe that most of this can be attributed to the amount of pain she must be experiencing, and I have no experience with that myself. I also have no experience with Methadone treatments, though I have a pretty expansive knowledge of addiction (and many expert resources available to me).

I bear no ill will towards her and feel nothing but a desire to be supportive. I just (apparently, by the lack of progress) don't know how to do that properly. I need help understanding what she is going through and how I can do more or less of something to make her more successful. I'm sure there are many schools of thought around these issues, and I've been purposefully general so as to cast a wide net at the range of possible solutions. I reach out to the community to help me learn about her/my options in the hopes I'll get some good ideas of what I might try.

Thanks for reading this rather long description.