HI Hun
Ive read your post and one thing jumps out at me, and thats the loss of your child, I am really sorry for your loss as I do know how devastating this is, I too have lost a child, and it never leaves you.
I wonder if you are at a point in your grief where, if you allow yourself to let go of your ex and the last vestige of that relationship, then you effectively let go of your late child too and the life he inhabited with you both, as he is the last connection to that old life for you?
I wonder if the grieving process for you is stuck at some point, which is not enabling you to go forward. Remember you have two losses here your child and your relationship you had with your ex. I think you may have combined the two and ground to a halt, where you are enable to make clear distinctions between accepting the painful truth of your ex moving on ( bastard ) and the on going pain of the loss of a child. One you can move on with, I too have an ex, and he fucked around with my best mates whilst in the hospital with her and she was terminally ill. The loss of a child never goes away, time will smooth the edges babe, but keeping hold of your ex as a friend will not salve the pain for you, it will just add to it. Change is hard, particularly when there might be a bit of empty nest syndrome coming up as well, theres a fear maybe of being left alone, and no one will blame you for clinging on to whatever you can hold on to.
The awful truth is you cannot stand still as life demands we move all the time, you have a unique opportunity to start another phase of your life, its ok to be happy, you carry the memory of your son with you always every second of the day, think what would he be saying? would he want you to go forward and live life for him?
I hope you find a path that allows you to do this.
all the best GP x