transition person
A transition person is the first persion you date during and/or after the ugly ending of a long term serious relationship, or possibly a drawn out death of your partner. They comfort you through the pain, self doubt, insecurity and other difficulties. Then, as you heal, they are a reminder of that terrible time and pain, so you gently ease them out of your life.
She was the one true love of my life, but I was just her transition person after Jason.