Hello AntiqueSinger. Thank you for your post and, no, you weren't crass at all! You have said what a lot of other, equally wise, people have said and I do believe it will turn out as you say. You're right - it will take time for me to get to a better place (as in happy, not dead!) and I just need to keep going forwards. My hope is that at some stage in the future I can come back to this thread and bellow, "Oi! AntiqueSinger! You should have bet £500,000!" 
Oh FantasticButtocks your posts have helped me so much. Other posters have commented on your great advice as well .... you could find yourself in the running for a place on the Mumsnet Royalty podium.
Your post made PERFECT sense and you were spot on with regard to the DCs' reactions and feelings. It is so good to have others' views on this and it is so reassuring. STBXH never really had much idea of what made the DC tick, especially DD1 (who is improving a lot since he left) and DS. He got on well with DD2, but she pretty quickly learned that the way to behave to keep him on board was not to butt up against him, but to distract him and get round him without any conflict. I think the fact that their relationship is OK and that she is still seeing him, says more about her character than his.
I am going to copy out your final paragraph about mature behaviour and keep it on a post-it note nearby. It is so perceptive and encapsulates everything that has happened to us.
I guess there is no telling how DCs will play their part and STBXH is really missing a trick if he thinks there will just be an initial period of the DCs being in a huff which is how he actually described it, before coming round and being OK with him and OW. I was thinking of somebody I knew who left her DH for OM. Their DCs were late teens at the time. She and her DH divorced and she has been married to OM for about 10 years now. One DC is getting married this year and has invited her mum and dad and the dad's new wife (they met after the divorce went through), but not her mum's DH. The DD still harbours ill feeling towards the OM after all those years and it is now a very sad and difficult situation for the DC, the mum and her DH. But I guess the DC is showing how she feels and that it won't always be a case of forgetting, never mind forgiving. My STBXH stressed over and over to the DCs that it was me he was leaving, not them, and that he wants to have a good relationship with them. I don't think it is really that simple in their minds...
Adarajames your post made me cry a bit, but in a good way. I was feeling quite low and down before I read it and your lovely words really gave me a boost. I hope I always manage to keep supporting the DCs - I love the parachute images - and that in 1 or 5 or 10 or however many years down the line, they will still remember and always know that I was there for them and helped them willingly and with no expectation of payback, gratitude or entitlement.
DD1 is coming out of her shell more and more and although I am still holding my breath, I think that things may well turn out OK for her. She has accepted an offer from the uni in our home town and is planning to share a house with one of her friends who works and some work colleagues who are also students at the uni. She said that she is happy to stay in our home town now that DH has moved out; previously she was looking at other cities and, quite frankly, that worried me. She is going to use the time between now and then to concentrate on her mental health and to take on some extra shifts at work to build up some funds. On her application form she had ticked a box stating that she had mental health issues and the uni phoned her to tell her all about the support they could offer in all different areas, including counselling. She has arranged a meeting with them on Monday and will have a chat about it then. Oh, I so hope it all starts to work out for her. Positive vibes all the way.
I have already gone NC with STBXH because the thought of seeing him, hearing him, receiving texts or emails is still too painful and upsetting. I can't even be in the house when he picks up DD2 and he doesn't come in; he sits in the car outside and DD2 goes out to him. I either go to a neighbour's house or a friend's or sit in the bathroom. I don't for one minute think he'd ever come in and beat me up or anything; I just can't even bear to have a glimpse of him. It's horrible, but it's the only way I can cope with it. I have always found it very very hard to think that somebody is angry with me or hates me and, if I feel that they do, I tend to hide away until they have gone. A bit like if I can't see you then you can't see me. Hmmmm, makes me sound a bit strange to say the least.
And you are a baker, you say? Cake?? Well, please feel free to fire up the oven on my behalf and send some this way. I can always freeze any surplus. And if DD1 moves out in September I have a whole room that can be given over to storage.
Thank you everybody for all your help and support. 