If you are a separated or divorced Catholic and you have not remarried without a decree of nullity (annulment), you are welcome and encouraged to attend mass and receive the sacraments. The Catholic teaching on divorce and remarriage is that all marriages are considered to be validly sacramental unless proven otherwise through the annulment process, therefore, remarriage without a decree of nullity would constitute a person having two spouses, which is immoral. If you are in this situation you are not permitted to receive the Holy Eucharist, but certainly are welcome and encouraged to attend mass because you are still an important part of the Catholic Church, the body of Christ. If you have remarried without an annulment and would like to be in full communion with the Church, please talk to your parish priest about the possibility of initiating the annulment process.
An exception to being separated or divorced and being able to receive the sacraments would apply if you are someone who deliberately destroyed your marriage by divorcing for selfish reasons, such as abandoning your spouse for another relationship, a career, or to be free of the responsibility of being married. There are many reasons why spouses divorce, but if you find yourself in the position of having willfully abandoned your spouse for immoral reasons, it is necessary to discuss your situation with a priest before receiving any of the sacraments of the Catholic Church.
If you haven’t remarried but are in an physically intimate relationship with someone other than your spouse, you must receive the sacrament of reconciliation (confession) before taking Communion, and this is something that applies to all people, whether they are married, separated, divorced, never married, or consecrated, and not intended to single out divorced men and women.