You do realise that though the om may be willing to take on your children that they are not likely to just be able to accept him? They are going to be angry, confused ,upset etc. They are just not going to suddenly accept a new man in their lives trying to act like a father, they already have one of those who they obviously love.
If you do decide to seperate from your husband then I suggest being with your children alone without the om.
But I feel you should make a go of your marriage. The grass is not greener on the other side ,and at the end of the day being with the om full time is going to be completely different to the affair. The excitement won't be the same.
He may be a good man and has brought up someone else's child, but what he is doing now is wrong. Does he not understand the children will take a long time to get used to him, maybe not ever. They are not stupid and they will blame him. And your children don't need another man to bring them up- THEY HAVE A FATHER.
Maybe you could try having some counselling either with your husband or on your own. This may help you talk everything through and put some perspective on it all.
If you cannot work due to childcare how the hell do you find the time for an affair? Perhaps all the energy could have been better used to get a job. Oh, and to be honest your children would have coped a lot better with a mum who works than they will if their mum decided to leave their dad as she has been having an affair.
I am sorry if I am harsh but you do not seem to understand the total devastation that you could cause which is always going to effect the lifes of your children, as well as their dad. Could understand it if you had tried to amke the marriage work, but while you have supposed to have been trying you have continued the affair instead of stopping it to give your marriage a chance.
Hope you can decide what you want with your life