If I remember a mortal sin I honestly forgot to confess before, does this mean I have not been forgiven? Or can I still receive communion? What’s the law on this?
Yes, you have received forgiveness for those sins. However, you are required to confess your sins next time you go to confession. With God, forgetting someone isn’t a crime, as long as one makes an honest effort to prepare well for confession but yet forgets to mention a mortal sin; this isn’t a sin and should not keep one from approaching the Eucharistic table.
However, if one willfully withholds a sin, none of the sins he confesses are forgiven. He in fact is guilty of abusing the sacraments; trying to cheat forgiveness from God or what? Approaching the sacraments presupposes openness to God; honest openness that shuts no doors to the permeating and liberating light of God’s mercy.
Obligation to Confess
“A member of the Christian faithful is obliged to confess in kind and in number all grave sins committed after baptism and not yet directly remitted through the keys of the Church nor acknowledged in individual confession, of which one is conscious after diligent examination of conscience”(CIC 988:1)
So, if you didn’t withhold the sin at confession, you should be able to receive communion provided you resolve to confess the sin at your next confession.
“Confession to a priest is an essential part of the sacrament of Penance: “All mortal sins of which penitents after a diligent self-examination are conscious must be recounted by them in confession, even if they are most secret and have been committed against the last two precepts of the Decalogue; for these sins sometimes wound the soul more grievously and are more dangerous than those which are committed openly.”
When Christ’s faithful strive to confess all the sins that they can remember, they undoubtedly place all of them before the divine mercy for pardon. But those who fail to do so and knowingly withhold some, place nothing before the divine goodness for remission through the mediation of the priest, “for if the sick person is too ashamed to show his wound to the doctor, the medicine cannot heal what it does not know.”CCC 1456