I found flash drives in my fiancé's stuff: one had a hidden folder of photos of family, wallpapers, and a folder named "Girls" with half-naked pictures of women -- both anime and movie star. Another flash drive had a New Folder with mixed pictures both half-naked anime photos plus photos of us.
I confronted my fiancé about it, and I don't understand his explanations or his denial that the pictures aren't his. Why would he still have the Girls folder after our being together for about ten years and only recently engaged? We have spoken about him watching women and how it makes me feel, to which his response is that he watches, but his mind is elsewhere.
A couple of years ago I found porn stored on his phone and that was quite a storm.
What should I do? How do I forgive? How do I forget? How do I trust, when it feels like nothing is changing? How to do the right thing? What is the right thing?
If he had a problem with pornography in the past, I suppose it is possible that he forgot these particular folders since they were not obviously labeled; however, you can check the dates on the files and see when they were last changed. The more likely answer is that he still has a problem with pornography, but had gotten better at hiding his sin.
Forgiveness comes when he repents of his sin (Luke 17:3-4), but it looks as if he doesn't wish to admit that he is sinning. When forgiveness does come, it won't mean that you'll forget, but rather that you will treat him as if it had not happened. But the reason you are currently struggling is that you are trying to forgive when there hasn't been any repentance on his part.
You can tell him that he has to change and that you'll not be marrying him until he does change. But understand that if he doesn't change, you'll have some hard choices to make.