I have heard horror stories of people who sneak off to GICs (Gender Identity Clinics) or start self-medicating without a single thought for their partner. This I consider to be extremely selfish, as they should know, as soon as you know. At least have the decency to allow them to seek a new partner should they wish to. “But I love them”I hear. I am sure you do; and they love you - but it doesn’t mean they want to be married to someone of the opposite sex to the person they first met.
There is no kind way of breaking this news. It is going to cause tears and heartache on both sides. While your mind is full of how good your life is going to be once you have made the change, the partner is struggling with many things. Embarrassment, deeply hurt, worried how they are going to manage the children, pay the mortgage/rent and tell their friends and family etc.