I've masturbated to incest porn for more than a decade and now I can only get aroused by it

Sorry to be "that guy" but...was it REAL incest p0rn (ie; only available on Onion sites) or make-believe\fantasy content?
if it was fantasy you should take apart the mental memories and dissociate the "kink content" from the "actual photo\video". Like...its just a random photo with a bad fanfic? I hope my insight helps heal.

 

Well, then just tell yourself you werent aroused by the women per-se, but by your own bad mental fanfiction
I think mocking the root cause of your own arousal is a great strategy.
Men can tolerate being mocked, but few can withstand the urge to laugh.

 

Its normal to not get aroused by anything else when you have got your brain used to big shots of dopamine with incest porn/roleplay which is your fetish.
All you need to do is refrain from watching these porn, reading it abt or roleplay or anything with respect to this fetish. Completely remove it off your life for atleast 3-4 months (varies from person to person).
This should re-wire your brain to start getting aroused when you are with another girl.

But this isnt going to make your fetish go away. For most, this fetish starts due to unstable relationship with their parent (not the case for everyone though). If you think you fall under that category, then try to get some help and talk abt it. No guarantee it might help making it go away but you can understand more about your fetish and about yourself in that process.

For most of us, our fetishes are related to what we fear, long for, our insecurities, our past trauma and much more. So try introspecting to understand more abt it.