Welcome to Beyondblue, and thank you for sharing with us.
I can hear the hurt in your post, and I can imagine that this has brought about so much confusion for you.
Sadly, there is not one reason to explain what your wife has done. Only she will ever know the real truth of her actions.
The divorce process will be tough. The fact that you moved here some time ago, have you developed a group of friends or close colleagues who will be able to support you? Losing a loved one through separation is just like any other grief. You mentioned that you had suffered depression for some time, are you still seeing a Psychologist/Psychiatrist? It might be worthwhile booking in some sessions so that you can at least talk through how you're feeling.
I'm really pleased that your son has decided to live with you, he obviously believes you are a wonderful father. Your focus needs to be on you and him now. Does your son have a friend that he talks to about his worries? He might need a bit of counselling as well.
The fact that your son is living with you will benefit you in terms of how the marital assets are divided. Have you been able to source a good divorce lawyer?
As hard as it is to imagine, there will be life after this, and yes, you've picked a beautiful country to spend the rest of your life in.