When you're in a downward-spiral, it is immensely difficult to think of another person. It's not that you don't care about their well-being; it's just that your first priority is self-preservation. By its very nature, addiction renders you very self-absorbed.
Addictions are safe-havens. There's always some benefit that you're getting out of it, even long after you've come to terms with its destructiveness. Of course, its a complicated business, especially when your partner gives you an ultimatum that forces you to decide which is more important. Problem is, addiction is addiction and it's not just a battle, but it's an outright war once you're on the path to recovery. It's scary, it's hard, and most people rather return to what's familiar than brave the unknown or the difficult. Most people with addictions also have self-esteem issues, and do not have the drive or the belief that they can recover, which tangles the knot further.
And of course, once the loved ones start retreating, who or what do you think the addict is going to turn to? That's right. Whatever is safe or familiar; the quickest way to relieve their discomfort.
Still, as a partner to an addicted party, I think its important to know that it is not your responsibility to get the person out of this mess. You can help as much as you can, but once you hit that unhappy threshold and you physically, mentally and emotionally know that you cannot take it anymore, you're not obligated to stay. You deserve happiness; you don't have to resign yourself to a life of being someone's nurse or parole officer. If the behavior does not change, especially the person is not making any effort to change it, that's probably a good time to really think through what sort of a future you're going to have with this person. There's only so many chances you can give. Eventually, you have to realize that this all stops being fair to you.
In your case, it appears as if you are extremely unhappy and you're approaching that threshold of no-return. In my opinion, if your partner is still not putting in the effort now, there's no guarantee that she will in the future. It sounds like she makes a lot of excuses for herself because of her "cowardice." Maybe with you there as her safety net, she feels too comfortable to make progress. Stepping back might be a good idea to see what she does... even if its only to help her get mobilized.
But again, I'd like to stress that you're not obligated to stay just because you've been in a relationship for so long. If the relationship is toxic, there is no point in being a glutton for punishment.
Good luck.