@Scared17,
From, 15, 38 years you two are best friends in your own way, you have to be having spent that many years together.
The "affairs" don't cook for him, clean for him, help pay the bills. He in my opinion would find it difficult to manage life now on his own. And, he doesn't want to lose his best friend. Don't you think he should have thought about that, "all" the way through this marriage? It seems to me that he wants a one on one but, enjoys the attention, flings, affairs it's not like he's only done it once.
One month isn't enough for him to realise your sincerity he needs to dry out and see what life is without you.
He needs to go to counselling himself, to ascertain what it is that makes him "need attention" from others.
Those would be my suggestions and in addition, go spoil yourself, do your hair, make-up buy a dress. Look at yourself in the mirror and see the beauty and confidence because you are still young and what ever road you end up taking, you have an identity that he can't take away.