Denil, he has not abandoned you yet, but has behaved irresponsibly . I myself have been too 'understanding' towards my late and former husband and so I say to you from what experience I have that you need to express to him your anger and horror at his behaviour. You will know if he possesses any saving graces, most people do. And you yourself need to decide whether or not your trust in him survives sufficiently to continue as his wife, whether or not he has other qualities that you love.
It seems that at least he feels remorse. You should express your anger but I get the feeling that you would like to preserve your marriage, and those are not mutually exclusive, rather the opposite. Some middle aged men feel that their lives lack the excitement that they deserve to experience! It's not difficult to fall in love with an exciting other woman and the silly man will often try to rationalise his affair. He needs to know that his affair affects you deeply and makes you angry, afraid, and grieving. Will he consent to go for counselling?
On a practical level you need to see a lawyer, unless you already know how you stand legally if the worst comes to the worst.Perhaps best not to tell him you are consulting a lawyer, as this could be ammunition against you, as happened to me.