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This isn't selfish, this is how you feel and you have the right to it.
In our experience around here we've learned that cancer REALLY makes people who they are, and it ain't always pretty. Get out. You will feel guilty as hell for it, so get out and get counselling because you need to find out why being a doormat has more value to you than being a caregiver to yourself. Really. Get counselling.
Abusers don't change, I'm sorry to say.
Good luck, and if you can, let us know how it's going.