How does a relapse happen? it starts way before the actual relapse, from a few days to whole weeks!
That's how it happens, step by step:
1- After the previous relapse, you're drained, guilty, lost and depressed, while your porn loving nerve endings are drunk on porn dopamine, resting and feeling happily saturated. All the while your sexual energy is depleted.
2- A few days later, your sexual energy starts to refill, that triggers your porn loving nerve endings to want the dopamine from porn (which is what you've taught your brain to understand what sex is, and normal people want sex from time to time according to their libido). When that starts happening, your brain starts flashing porn fantasies and images to your visual area, starts ordering you to stare at random women, starts making you like minor stimulants and seek them (and in my case, it also frequently makes me dream of erotic shapes or situations).
3- Next, you seriously begin seeking minor stimulants. You exploit images of women you see on TV or the internet (that people see as normal images) and you start imagining them as some sort of softcore porn. You really start liking that and pursuing that, while you keep convincing yourself that they're not porn, and you're still abstaining, still going strong on your awesome long streak.
4- All the while, you start masturbating (not to porn), in order to satisfy parts of your cravings. You think it's a good idea (because it keeps you away from wanting porn for a few hours). But the reality is, they make you want porn more, because the dopamine your porn loving nerve endings get from masturbation are not half as much as what they get from PMO, so they BURN with urges..... things get out of control.
5- More minor stimulation.... masturbation increasing in frequency.... Your enormous fear of falling into the next relapse intimidates you... You either fall at this point or...
6- In a moment of sheer terror from the imminent relapse, your filthy brain starts dropping ideas the likes of: a- You can't get away right now, you'll have to postpone quitting till something magical happens. b- You have more time, you had a good run, you can award your perseverance with JUST ONE relapse, then you'll be fine afterwards. c- Relapsing can be a good idea since it will make you more resolved to quit next time, just look at softcore and don't binge this time. You start liking those ideas because they give you some false safety and confidence which makes your fear subside, and BAM! you're back to where you were.
Side note: I'm not saying that since step one starts a relapse is inevitable, it's just harder to control the progression as it progresses. I'm at step 2 now (no I don't stare or seek minor stimulation, but I got an erotic dream that started me contemplating again). Now that I'm aware with the dirty game my mind plays, I'll arrest that progression NOW and HERE. No more stupidity. NO PMO. NOFAP!!