Thank you for your replies everyone. I have been reflecting between daily life/work.
@FudgeBrownie2019
He even has you believing his bullshit that staying faithful is hard for him.
Oh, that's not something he's actually said, that was just a glib way of me putting it. I think it is just a defence mechanism for me to try and be glib about it - maybe it will hurt less that I'm cast over in favour of some random woman? 
Of course I don't want to tolerate cheating. I'm just having trouble with accepting that I've picked badly again.
I've been married before, married far too young at 22, and he became abusive. Divorced at 24. And now what, divorced again before I'm 35.
@soontobe60 - I'd tell her exactly what everyone is telling me.
I can't bear the thought of potentially only having my children 50% of the time though. They're my world. Its bad enough having to leave DC to work 
@doritosdip
Has he made any changes to stop his behaviour like getting rid of his smartphone etc?
He'd never get rid of it. It was generally done over FB messenger. I did have a look on there when he was asleep the other night but there was no sign of anything more. But is he just getting better at hiding it? Possibly so. He also appears to be an avid user of Snapchat and Kik. But I can't see from his phone what he's up to on Snapchat. There was anything on Kik, just empty 'conversation' boxes. I don't know how they work though, I don't use either.
I have thought about couples counselling, as I've never been able to get an answer from him about why he feels the need to chat up randoms online. He knows he's destroying my trust each time and each time he swears he will never do it again. But even if he doesn't, I'm struggling to put to the back of my mind that it's happened several times before.
@AttilaTheMeerkat - thank you, this really has given me something to reflect on.
@Waxonwaxoff0
I'm a child of divorce and it really isn't the world shattering experience some people seem to think it is. It's an exaggeration. 50% of children come from divorced families - trust me, we cope fine.
Thank you. I am too. I always wondered why they didn't divorce sooner, because of how they argued.
@GrumpySwivelHead
He does. He never even has an answer to why or whats the issue between 'us' that he has to go elsewhere.
In an ideal world, he'd completely stop it an I'd be able to trust him again.
He won't leave me for one of them, I'm pretty sure. He has an easy life really. I do most things because it's easier/he doesn't help with the mental load much. Or actual load tbh. He's making an effort received though I must admit - cooking, spending time with DC, cleaning etc.
Wow I'm pathetic. I can't stand the thought of not having my DC there every day and having to 'share' them with him.