I'm sorry you are feeling so down but please be reassured that you will regain your self-esteem. It just takes a little time and effort. I promise you. I have rebuilt my life twice now, once widowed and once divorced. I'm now happier than I've ever been.
If starting a fitness regime will help you to feel better then go for it. What's stopping you?
Take it one step at a time and you will get where you want to be.
Couldn't agree more about the 'remaining friends' nonsense. The woman I married, the mother of my children, she was my best friend. Why should I want to be friends with this stranger who is divorcing me?