What does one do if one has committed a mortal sin and is unable to confess?
From The Catechism of the Catholic Church:
1452 When it arises from a love by which God is loved above all else, contrition is called "perfect" (contrition of charity). Such contrition remits venial sins; it also obtains forgiveness of mortal sins if it includes the firm resolution to have recourse to sacramental confession as soon as possible.
If one makes an act of perfect contrition, then, and intends to confess as soon as possible, may he receive Communion before he can confess?
From The Code of Canon Law:
Can. 916 A person who is conscious of grave sin is not to celebrate Mass or receive the body of the Lord without previous sacramental confession unless there is a grave reason and there is no opportunity to confess; in this case the person is to remember the obligation to make an act of perfect contrition which includes the resolution of confessing as soon as possible.