I have exactly same problem. I have so many triggers from everyday things that I cannot avoid, the head porn just keeps popping up.
I've managed (so far) to get over this by simply distracting myself. I will think of something else. Anything! Anything at all that isn't related to the porn! I'll think about something I saw on TV or on the news, my next holiday, my last holiday, my bank account, the book I'm reading, what I'm going to wear tomorrow... the list is infinite but you do need that little bit of discipline and willpower to make yourself do/think something else. I've started playing games on my phone a lot (you won't believe how many levels of Candy Crush I've moved through since giving up PMO!)
Sometimes I'll find that I dwell on the head porn without realising but as soon as I feel even the slightest bit of excitement I get myself out of it. If you're finding you're getting hard or even semi then I think you're letting it stay too long. Don't put yourself down about it though, just tell yourself it was a close call and think or do something else.
I'm only a month (almost) PMO free so maybe I'm not the best person to give you advice but for me having lots of distractions has worked really well. If I'm feeling really horny I'll try and think about a real life woman (an ex girlfriend who I liked) but not in terms of porn. I'll just think about her holding my hand or think about some romantic moments we had together but not sex. I'd like to think that it is helping create the good connections in my brain - i.e. connecting pleasure with normal interactions.
Good luck!
