Just like anything that jacks dopamine. Your brain gets used to the elevated levels and craves more, you need more and more to feel the same way so the behavior escalates. Things that normally raise dopamine don’t even produce enough to register so everything else becomes less enjoyable. Thus the behavior becomes compulsive. Far from being a fringe phenomenon pornography has been proven to be extremely (like illicit drug level.) addictive.
No. You can get past it, as indicated by the fact that you get through the most difficult part (the initial phase.) Repeatedly. You’re having trouble with the long term struggle which is totally normal. It might help to read up on how other people broke through that barrier before, or reach out to a resource group for help.