I am so so sorry, faff. I know what it feels like, I promise. Please please find an individual counsellor and please please think about who can support you, it doesn't need to be parents, or even old friends. What amazed me at points in my life is who shows up, sometimes quite casual friends become much much more if you let them into a traumatic time and it's amazing how many people have been through similar.
Please don't close your mind to asking for help and real life support, though we will be here for you too.
It does not surprise me in the least that you say your whole adult life has been arranged around him. This is true in the vast vast majority of affairs. There is an old saying around here that people are infidelitous when they are not GIVING enough to a relationship, not when they are not GETTING enough from it. Whenever someone invest a lot in something, they inherently value it more, this is true economically and emotionally. When someone invests less, either because they can or they are afraid or any number of things, they inherently value that thing less and will be more likely to betray it. It's called, commonly, taking things for granted!
It would not surprise me if when you start to think about it, your DH has shown entitled behaviour in other areas too, not because he is a villain, but because he has a tendency to compartmentalise, crave validation, or has had a lot of success in areas that offer him a lot of praise and status. He probably also manages to make himself the victim in things when he is retelling them ('she has cause a lot of trouble for me' etc) and that he is often somehow central in things, even without realising it. There will be traits that allowed this infidelity to happen and they wont' just apply there.
Faff, I'm not going to push with speculation or things at this point which will hurt you more, but I ask that you keep your mind completely open and try and surf this a little before making decisions. You will get stronger at some point. The anger will kick in, and that is the time to use that adrenaline. Right now you need to think of yourself as bereaved and make no major decisions. Because I promise you, they will not be based on anything solid. Of course he's sorry, whoopdidoo. He'd be a fucking psychopath if he wasn't sorry.
As you say, yesterday you thought he would never cheat on you etc. You do not know what you feel or, to be honest, what has actually happened. You do not know. You are going to survive and beat this not knowing and I promise you will get through this. But please do not continue to make him the centre of things right now, especially your sanity, support and source of beliefs. He is not your supporter right now. He is fighting for himself. Please trust us.
I promise people surprise you when you reach out with vulnerability. I know it hurts. It is awful right now. It will not always be this bad.
Much love.