Not good... not good at all. I understand what you are going through.
I cannot even begin to understand the kind of person who would do such a thing.
Alright... I'm going to offer a different opinion here. There is a famous Lao Tzu teaching that goes: "Watch your thoughts; they become words. Watch your words; they become actions. Watch your actions; they become habit. Watch your habits; they become character. Watch your character; it becomes your destiny."
I have always been vehemently against cheating. I have also always been quite righteous and fiercely moral. I have also been guilty of judging people instead of actions. What N did was utter shit - on so many levels. For sure, if I found out that my partner had done this to me, I'm not sure I could forgive them, even if I could understand it eventually.
However... what I will say, as a differing opinion, using Tzu's teaching, is this:
N had thoughts of other women. Eventually, for whatever reason, they became actions. Then, they became a habit. His entire character (and thus, his destiny in your relationship) is now up for assassination. And so it should be - but not just by the two of you. If he has a shred of decency, he will have already been assassinating his character over this.
Another thing to consider is that, whilst all lies are lies, there are different types of internal responses to telling a lie. Some people feel nothing because they are in so much denial; some people feel awful, because they have a strong conscience; and some people get a sense of thrill, because they can get away with it. Just because N lied, it doesn't mean that he felt thrilled about lying. If he really is a wonderful guy, he more than likely felt either, or both, of the first two reactions. I do think that whilst some people are just, in short, complete shits, others are simply weak, cowardly, and this is why they end up lying.
When someone in a couple cheats and they decide to go to therapy to try to work it out, the relationships that grow to become even more successful and happy than they previously were are the ones where a deep level of *understanding* and forgiveness has taken place. This takes a lot of work. You cannot forgive someone without understanding why they did something in the first place.
Whether you can personally forgive him or not is down to you. You wouldn't be 'wrong' for not being able to. You simply may not emotionally be able to.
You said that you are looking for advice, so here is the advice I have if you do think that you would eventually like to find some way of moving past this with him.
Firstly, accept that it will be a long road. Resentment and doubt will take a long while to be reconciled. Secondly, realise that he is not a different person now - but, instead, that you have a more realistic view of his whole persona and history. You will never fully know him. We never fully know anyone. If you realise this, you may realise that this *might* pave the way to something even better than what you have already had together.
I would personally ask for a set amount of time to achieve space, silence and deep thinking - on both of your parts. A week, two weeks, or whatever you need. Negate the panic and grip of him desperately trying to call you. Essentially, send him off to think about the impact of his actions and give yourself the space to breathe.
Then, I would arrange to meet - in person. Allow yourself to hear him out. Create an environment where he is able to, and knows that he absolutely must, be completely truthful.
Once you have his reasons (and you should try to listen to them quietly), you should tell him how you feel and what your (understandable) concerns are for the future.
Over the past two years, I have changed my opinion somewhat about topics surrounding cheating, lying, selfishness and cowardice. Those four things are the things I detest most in relationships and the traits I cannot stand in people. However, we are all human. We all make mistakes. We all constantly change. Sometimes, we do absolutely, utterly idiotic things. Sometimes, we live in denial and excuse our behaviour (he was likely doing this by keeping the videos).
I had a bit of a revelation recently. What my GF did wasn't, by any stretch of the imagination, anywhere near as dishonest as this. It was a minor infraction; but a lie, nonetheless. She had assured me that she was only friends with someone; but, soon enough, he did something public that might reveal the real nature of their relationship to me, should I stumble upon it. Before I could see it, she came to me in tears, petrified of my reaction and told me the truth about what was going on between them.
My first internal reaction was the dark place we all go to - the thoughts of "I'll never be able to trust them again", "how could they lie to me?", "what does this say about their character and personality?" etc. I felt judgmental and uneasy about the actual *things* she had done with him - and also the lie itself.
Instead, for the first time in my entire life, I stopped myself from freaking out, bit down hard on my tongue, breathed carefully and listened to her. She went through stages of justifying her actions ("This wasn't cheating, I wasn't lying to you, I just needed to work out what I was doing with him and come to terms with it before I told you, you ask me not to share details anyway in general." etc etc) .. to moments of accepting that she knew she wasn't being as honest as she could, or should, have been. By not taking myself to the dark place and focusing on *understanding her reasons*, it was much easier for me to process her *actions* and not apply them to her *character*.
Basically, she 'lied' (withheld) because she was scared to tell me, because she was living in fantasy land, because she was in denial about what she was doing in the first place, because part of what she was actually doing with him, she was a little ashamed of and didn't fit with her normal behaviours. And you know? I think all those reasons are valid. And I can see where I had played a part in creating an atmosphere where she did feel scared to tell me. Once we *start* doing something we know we shouldn't be doing, it's easier to carry on doing it than it is to come clean when the event is at its smallest. We dig ourselves a hole. I only hope that if I ever find myself in a moral dilemma and find myself fucking it up horrendously, that she will show me the same compassion and understanding.
So... there's my advice... I hope that it helps. ~smiles~