She wants some of what she can't have.
Will get it and move on or end up pregnant and causing problems
Just go n/c and move on with your life
Be a respectful human
For her to jump in the sack with him and then back to you, it's more than PTSD
She wanted to try him out instead of you and it didn't work out. Now she wants you again and when you say the wrong word, she will bounce.
Vicious cycle you don't deserve
Don't do it
She had a chance and left you for him. Slept with him and it didn't work out. Now she wants you back. You might have the feelings now and in the future it will always be in your head.