The first part of this post sounds so so similar to my boyfriend/ex/mess situation, it’s almost exact beside the part about not watching porn and the incident with a minor. Please please do get specialist counselling, having gone through discovering my partners secret sexual life (I was also sent screenshots) he then was remorseful and full of shame, for a while, then the cycle of addiction rolled on and he did more, worse things ( if you google the cycle of sex addiction it will help understand where your at) Its only once he was ready to admit that he was sexually compulsive and was using female attention as a buzz and online and offline plus ever wilder stimuli needing more and more (even though we never stopped in terms of sex and were very active) that he began to be able to take steps to admit it was an addiction, it was out of control... and frightening once I found out where this was taking him. I hope sharing this is helpful, your best avenue IMHO is to go to a counsellor and tell them exactly what you have said here, they are trained to understand and help, the damage this does to your girlfriend is awful (not to mention the very worrying incident you speak about, which is extremely serious) and you are right to want to address this. It won’t go away just because you feel sorry, as you have realised, it’s a compulsion. Unless you start to work on yourself it will return and even if you do start therapy it takes a lot of fight and help to change the brains habits for the long term.