"To grieve the loss" means to feel deep sadness and sorrow over the absence or disappearance of someone or something that is very important to you. Full definition
Even if you both recover from infidelity in the future, you are still grieving the loss of what your relationship used to be. (marriage.com)
That can be tough if you are still grieving the loss of your marriage. (grieflink.com)
Providing a safe environment to express feelings and a helping hand can make a difference in this person's physical and emotional health while grieving the loss of a marriage. (oureverydaylife.com)