Hi Jilly,
Welcome to SR.
In short, no you're not wrong for wanting to leave. Sure, you've put 8 years into him and everyone wants a 'return' on our investments. But unfortunately, the disease of addiction doesn't work that way. You're actually way ahead of the game when it comes to not bailing him out. When we bail our addicted loved ones out (literally or figuratively: jail, due bills, food money, etc.) it is sustaining their illness. It makes us sick and keeps them sick. If they don't experience every consequence of using their drug of choice, every time they are rescued, it is another moment that they are allowed to remain in denial about their addiction.
Regarding his back injury, its easy to feel guilty when the addict also has a legitimate physical/mental condition in addition to their substance use. But you know what? Once someone is addicted to heroin, the only pain they're running from & trying to relieve is the pain of withdrawals. His back pain is but another excuse to keep using.
We all have pain, and there are many ways to deal with it that don't involve breaking the law, risking HIV and other bloodborne illnesses, and using a drug that is of unknown purity/inactive ingredients/source day in and day out.
I don't know from your post if he is an IV Heroin user. But I want you to know that regardless of his route of administration, if you are in an intimate relationship with him, you are at risk too.
I'm proud of you for sharing your experience.
If I can, I will suggest a book by Melody Beattie called "Codependent no More". Its really for anyone that is affected by another's addiction. Perhaps you'll find things in the book that you can relate to, and tips on taking care of yourself.
Please, continue to share your story. You are perfectly in the 'right' for wanting to get out and be gone.
Its a higher form of love than sticking around and enabling (loving the addict to death), in my opinion.
And you're right.
You DESERVE (and CAN HAVE) SO MUCH BETTER.