“It’s so unfair… he cheated and I’m the one who has to suffer and pick up the pieces  and learn to trust again. I didn’t deserve it so why does he just bury his head and expect me to get over it?” Pete replies…

I really understand that it seems unfair and that you are the victim here – it’s how most people would feel and respond – and therefore you will find a lot of people who would support you in feeling like this – but this only makes it worse for you.
I’m going to offer you a different way of looking at it – and I do this with the intention of wanting to help YOU to feel better about the situation.
He cheated because HE is suffering.
At this point I will lose a lot of readers because they want to stay in the place of blame rather than seeking to genuinely understand what is happening here. Blame simply creates continued suffering for everyone. Understanding can bring freedom from suffering.
So, why is he suffering? He has been suffering for years – he was suffering when he met you. He has been suffering since he was a little boy. But suffering with what? Emotional malnourishment.
How do you feel when you don’t eat? First, it will be a little grumbling of hunger, but if it goes on for too long, we can feel really debilitated from not eating any food – distracted, grumpy, angry even. Our emotional systems need food too – but it has to be the right kind of food – unconditional love. If we don’t get enough of this emotional food, then we can become distracted, grumpy, angry even – it’s actually painful, but in a way we don’t really realise.
When we are genuinely hungry, we would eat anything to fill that hunger. Why do you see homeless people searching though rubbish bins for scraps of discarded food – because they are desperate. It’s the same when we are emotionally hungry – we would go to any lengths to get rid of that horrible hungry feeling – including sleeping with almost anybody else, believing that it will make us feel better. The truth is, that for a fleeting moment, he might have thought it made him feel better but the reality is, it wasn’t better at all – it was a false distraction from the pain of his hunger.
So, the victim view of this situation is that he did something to you – and that his actions said something about you. They didn’t. What he did was he went feeding from scraps of food from a waste bin to try and get rid of his own emotional hunger.
The truth is that he is in so much pain, he is blind to the truth about what he is doing. And this is why he appears as if he is burying his head – he is desperately trying to avoid any further emotional pain. He will feel guilty, embarrassed, ashamed and worthless, and all of these things just heap onto his already existing emotional hunger. The more you blame him, the more hungry he will feel and the more desperate he will get to get away from that feeling. This might well push him to try and repeat what he’s already done.
The situation needs understanding, not blame. Don’t focus on trusting him not to do it again. Trust that he is doing absolutely the best that he can. Do you blame homeless people for eating from public trash cans? No, you feel compassion for them. If you can find a place to see the truth of his hunger, maybe you can look on him more compassionately and understand he did what he did because of his emotional hunger. Trust that he can learn to find out the truth of this for himself. Trust that he doesn’t like the way he feels. Trust that he has the capacity to change if he understands what it is that is actually happening. Trust that he will always be doing his best, even if his best doesn’t look too pretty.

I am not excusing what he has done. I am hopefully helping you to genuinely understand the truth of it. Then maybe you can feel that this was not about you at all – it was all about him. He was emotionally hungry when he met you – so you have caused none of this. Maybe you can offer a gently hand of acceptance and willingness to walk alongside him if he is willing to go on a journey to learn he to get properly emotionally nourished for himself.