Addiction in general starts because someone can't deal with their feelings or the reality of their life. With any addiction, unless someone seeks help, it will progress as we need a better fix. Addictive behavior can be modified successfully if someone is willing to embrace the 12 step program, work the steps, attend meetings, get a sponsor, and be diligent about understanding he/she has no power over their addiction. I've been in recovery for a long time. Am I cured, no. I can always go back if I allow myself to stop taking care of myself.

Porn addiction is just that,pornography. Movies, books, magazines, internet porn. All of these present porn as the way sex is. Generally speaking, porn addiction can and often does lead to sex addiction.

Sex addiction may or may not include pornography, and is the more dangerous and damaging of the 2.
Sex addicts actually engage in risky sexual behavior with others. Affairs,prostitutes, picking up strangers, meeting strangers online for sex, same sex affairs outside of marriage as well as exhibitionism, public sex, rape, molestation,and other illicit and illegal forms of sexualality. Sex addiction is very often the physical manifestation of a pornography addiction.
The images do often stay with us for a long time. I have a 28 year addiction that I have been in recovery with for over a year now, and I have flashbacks a lot. They will begin to fade out eventually, but the trauma is always there, I am afraid.
There is no "cure" for sex and porn addiction. There is recovery and abstinance from it, and in many cases the urges and desires fade out after awhile, but just as an alcoholic or drug addict is always succeptible to re-use, so it is with sex and porn.It never goes away or is cured, only managed and controlled.