I am horrified.
If you were a bloke Cheshire you would have been torn apart on here.
Sort out the marriage, either save it or end it, but don't f*ck someone else.
And for the love of GOD don't introduce the F*ckBuddy to your DC.... 
END it with the OM. If, long term, it's meant to be, it'll be. But not like this. THIS is a disgrace, it's a mess and you know it.
Look at your life, take a holiday with DH and see what is what. Talk to him about your loneliness, find something to do to offset the void, and get your DH to look at his life with a more balanced view.
Clearly BOTH of you have to make changes to have any hope at all of making a go of this, but shagging some hanger-on is not going to be of any help to anyone.
It may be that the marriage doesn't survive, but it'll never have a chance when you have OM on the scene. How could you feel like saving your marriage or anything more than a cuddle with your DH if you are shagging some other bloke?
Imagine if your DC could sit and read this extract of your behaviour, how would they think of you?
Behave yourself, do the right thing by them and your family.
What will be, will be.