Being Divorced Isn’t a Mortal Sin

No, someone who tried to maintain a marriage but was divorced by the other party is not in a state of mortal sin.

The Catechism of the Catholic Church:

It can happen that one of the spouses is the innocent victim of a divorce decreed by civil law; this spouse therefore has not contravened the moral law. There is a considerable difference between a spouse who has sincerely tried to be faithful to the sacrament of marriage and is unjustly abandoned, and one who through his own grave fault destroys a canonically valid marriage (2386).