Hi Phoenix,
Try looking at all this another way if you can. Instead of the unfairness of it all, and focusing on the negatives (nearly 50, having to start again, how tough it's all been... – all true of course but dwelling on it won't change any of it) try looking at them and the situation you find yourself in, in a completely different way. You can't change what has happened. You can't change what they have done and how they behave now. But what you can change is your perspective and also your response. I know it might not seem like it a the moment, but you can, I promise you. I have been where you are.
Firstly, look at the way your H has behaved/is behaving and tell yourself you are now free of it. Focus on that word 'free'. The man you loved and married is gone I'm afraid. He only exists in your imagination. The reality of the person that exists now is a lying cheating scumbag who had an extra marital affair and continues to treat his wife of 13 years (you) appallingly. He also sleeps in someone else's bed every night - by choice. That's the truth of it, and he's certainly not worth your sadness or regret.
Secondly, regardless of what goes on between them, he's not going to tell you he's unhappy now is he? It could all be a sham for all you know – a total sham, or a partial sham, who knows and who cares? The woman he is with has got form - 'a trail of married men behind her' according to you, so he's hardly got himself a catch has he, and she's hardly got herself a catch with your husband. Just another one in a long line of errant husbands. He's a lying cheat and she's a lying cheat and the stone cold truth is they deserve each other. I doubt very much whether they will make each other happy, long term but for the moment he's strutting around like the Lord of the manor and she's walking around like Lady Muck and clearly getting a great deal of pleasure out of rubbing your nose in it.
Don't let them Phoenix. Look at them with the disdain they deserve. How pathetic they are really? The fact that they are even behaving like this, shows you the kind of people they truly are. You are so much better than them honestly. You are better than your H and you are better than her. I hate using cliches but please try to rise above it. They are not worthy of your sadness, upset, regret, envy or any of those emotions. You have wasted enough of yourself on these people, and now it's time to turn your attention to yourself, and your life, and making things better for you.
When you see her Lady Mucking it up the school, instead of getting upset, just smile to yourself at how pathetic she actually is. Let her see you smile if you want. That'll wind her up for certain. I know 'cos I've been where you are and that's what I used to do. Just a little 'smirky-type' smile to myself and an imperceptible shake of the head as if to say 'dear oh dear, how pathetic' and that doesn't half deflate their cockiness - trust me on that. 
.
They don't need to know that you are feeling like shit inside do they?
Even if you have to put on an Oscar winning acting performance, do it. Let them see/think that they are nothing to you, even if that's not true at the moment. Eventually there will come a time when they will no longer have the power to hurt you .. but that may be some way off, so in the meantime - wing it.
And while you are winging it in regards to your behaviour/demeanour towards them, use the rest of your time to do everything you can for you right now. Get fit, take up hobbies, pamper yourself, do things you love to do, get a spring back in your step, do a bit of strutting yourself!
Oh and one final thing. He's not your 'poor husband' . He's a lying, cheating scumbag who treated you appallingly and continues to do so with his cruel behaviour and you need to remember that at all times.