Well, the more you post, the less 'out of the blue' it sounds, OP. It sounds like you were both quite unhappy for a while. I left an eight year relationship in the same way and exP thought it was out of the blue, whereas to me it was on the cards for months, if not years.
ExP.later.saw the truth of that and thanked me for doing what he never would have 'had the courage for'. We're on good terms now, both happily settled with other people.
We didn't have children but really, my children is why I left, I couldn't let myself have children inside a relationship where we were 'just getting on with it' - I wanted to model something more fulfilling than that. I'm not stupid - I didn't want the fairy tale -but I did want something which enhanced my/ my partner's life, just as a base line.
It was a tough split- we both had periods of not wanting to let go, but it was ultimately the right thing for us both. I have a little boy and another child on the way, and the relationship they are coming into Isn't perfect but it is always growing, we all continue to grow inside of it. And I think that's what you need as a bottom line, really, that's what I was looking for when I left ExP thinking there was more to life than what we had.
There was no affair. I did go through a thing of really fancying another man but didn't pursue it cos I thought-correctly- that it was just a symptom of the fact my own relationship was in trouble.
I just wanted to add this and offer a perspective on what can end a relationship without an affair. And say that the more you post the more you seem to fit this picture. Seems like there is a determination to prove your DH is having an affair, which does read a bit obsessive TBH. My thought would be, does it matter at this stage? You sound finished with the relationship yourself, when you are talking about its reality.
Even fundamentally great decisions like this though are painful, it leave you spun out and feeling bereft of a future because all your plans were with him. So one step a time, I say, first get your practicalities in order- go to the CAB and work through the financial implications of the split with them. They'll be able to advise you of your H' s obligations and your own options for support. Then get back on th phone with your mate, tell her your plans and secure a bit of handholding. I'd also suggest writing a list of things you couldn't have done with him in tow that you might like to try, but it's early days...