I appreciate you sending in your situation and trusting me with a response. Having you cut your ex out of your life completely isn’t really an option. You have a child together and that child’s needs — at any age — is going to require some discussion throughout the lifespan. But very limited contact is probably a good idea for your own well-being. You need time to heal, and constant contact stirs feelings — even if it is brief.
I would begin by writing a letter to your ex-husband complete with all the thoughts, feelings, and disappointments you felt, as well as the joys and high points. Research shows that expressive writing is an excellent way to begin experiential grieving. It helps to organize and integrate the feelings you are having.
The letter is not one for you to send but rather one that is targeted with an intention. The letter is for you.
In keeping with the therapist’s recommendation to cut off contact, I would limit your contact to functional elements regarding your son or practical matters such as finances, etc.
Finally, I would become diligent in engaging yourself in social, creative, and intellectual activities. Start pursuing goals that will enhance your well-being and personal development.
These three things tend to help not only unhook from your ex, but also point you toward something more for yourself.