I hope this doesn’t look like I’m promoting my post, but there’s a ton of information from different people on the topic of porn and sex addiction in the responses here. Perhaps you will find the answer among their advice and stories.
I’m not sure why you need a “demon” specifically. Wouldn’t a deity or something similar work?
It’s not a popular opinion here, but one of the most effective approaches to addiction treatment (the 12 steps, if I’m translating it correctly) is based on the idea of God, for example. Also, many people have been able to completely overcome addictions thanks to faith in Jesus. Some may agree or disagree, but this is a fact I observe. I work closely with psychologists and psychotherapists who work in the field of addictive behavior, especially alcoholism, so I’ve seen such stories often.
There are also Asclepius and Panacea from ancient Greece.