Baba - if you read this, and I hope you do, please read it as it is intended to be read, as advice and not having a go. You mentioned that your dh has been dumped by 2 partners previously for other men, he also found one of his relatives hanging in the garage. But with you he thinks he has found happiness, he has no reason to believe that there is anything wrong with his marriage as you haven't mentioned anything. You have a little boy together, and from what you've said he adores his little boy and his boy adores him. To make things even better for him, you and him are trying for another baby - on film in fact!
Just think now how he would feel if all that were taken away from him! Let us just move you aside for the moment and concentrate on the other people involved in this, your lover's partner, your little boy, your husband.
You mentioned in other threads that the affair was over, so what started it again? You say you fell deeply in love with your dh, but now have fallen out of love with him and in love with your lover. I can see a pattern emerging here, do you really think that you will stick with this new partner? Is he worth giving everything up for? He is already cheating on his partner, what's to say that he will not do the same to you? He obviously cannot be trusted.
I have no doubt that tonight you are with him, and yet this weekend you are going to Glasgow to start filming 'Making a Baby' and hopefully to try and conceive with your dh. Does this sound very fair to you?
So my advice is this. Ditch your lover, he's a cheat so he's not really worth it. Confess to your dh that you are not happy with the marriage. Stop trying for a baby until you have sorted things out, it's not fair to bring a baby into this mess. See a therapy for yourself, because you have a hell of a lot of problems that will not go away by themselves. Until you sort out the problems within yourself, you will never be happy.
Hope you manage to take some of that advice on board. It is meant well.