Must we confess before Communion because God cannot be in the presence of sin?

Who says God cannot be in the presence of sin? Scripture shows Jesus in its presence. Perhaps you are thinking of "nothing unclean will enter" heaven (Revelation 21:27).

Receiving Christ forgives venial sins. As food restores strength, the Eucharist strengthens charity, which tends to weaken in daily life, and this living charity wipes away venial sins (CCC 1394).

It does not forgive mortal sins, so one conscious of mortal sin must go to confession first. Canon 916 says such a person may not receive the Lord's body without prior sacramental confession unless there is a grave reason and no opportunity to confess, and then he must make an act of perfect contrition including the resolution to confess as soon as possible.

Receiving in mortal sin without forgiveness is another mortal sin. Paul says, "whoever eats the bread or drinks the cup of the Lord unworthily will have to answer for the body and blood of the Lord" (1 Corinthians 11:27). The Church calls this sacrilege: profaning or treating unworthily the sacraments and what is consecrated to God, a grave sin especially against the Eucharist, where Christ's true body is substantially present (CCC 2120).