Sykes, I'm really sorry for what has happened here. You probably want to hear how he will come back to you and all will be forgiven and forgotten. But does he deserve you now? Not only has he hurt you, but your daughters too, if he loved you all that much, would he put you through all this? Yes you can blame it on the mid-life crisis, but I bet you don't go off with someone everytime you get PMT do you? It's an incredibly selfish thing to do. I wonder if perhaps he knows that you will wait for him, so in his eyes he has nothing to lose. He has a younger lover who at the moment is attentive to his every need, and a wife who is waiting patiently in case everything goes wrong with his lover. Would it bring him back to reality if you saw a solicitor and starting threatening divorce? You don't have to go through with it obviously, but it would make sense to see a solicitor anyway as you will need to sort out your finances, etc. If he realises that he stands to lose you for good, he may rethink his situation.
Finally, are you prepared to give him that chance? What happens if he does come back, how will you trust him? I know you want to protect your daughters, but surely they would be happier if the both of you were happy, rather than staying in an unhappy marriage. Kids do pick up these vibes and they will know if you are just going through the motions. Also your daughters may be looking to you as their role-model. So if you teach them not to put up with a cheating partner, they may just respect you more for it.
You are worth more than second best. You have put a lot into this marriage, how would he react if the tables were turned? Be strong and be brave, show him that you are not to be fooled with. Even if he doesn't come back, you will respect yourself more for it, and so will your children. You don't have to put up with this, so please don't.