- Full knowledge — This means that you know something is sinful at the time you commit it. For example, some children are never told that missing Mass on Sunday is a serious sin. Until someone explains this to them, they are probably not guilty of mortal sin.
- Grave matter — a serious offense. It would be almost impossible to make a complete list of serious offenses/grave matter; people keep coming up with new, creative ways of offending God. You need to have a well-formed conscience. If you are unsure whether something is grave matter, ask your priest in the confessional. Part Three of the Catechism of the Catholic Church goes into a fair amount of detail on the Ten Commandments and how they are violated. Read it.
- Yes, we lose sanctifying grace when we commit mortal sin. Sanctifying grace is the life of God in us. Mortal sin is rejection of God, and He will not dwell where He is not welcome. From the Catechism of the Catholic Church (emphasis added):
1861 **Mortal sin **is a radical possibility of human freedom, as is love itself. It
results in the loss of charity and the privation of sanctifying grace, that is, of the state of grace. If it is not redeemed by repentance and God’s forgiveness, it causes exclusion from Christ’s kingdom and the eternal death of hell, for our freedom has the power to make choices for ever, with no turning back.
Also from the Bible (Hebrews 10:26–31):
If we sin deliberately after receiving knowledge of the truth, there no longer remains sacrifice for sins but a fearful prospect of judgment and a flaming fire that is going to consume the adversaries. Anyone who rejects the law of Moses is put to death without pity on the testimony of two or three witnesses. Do you not think that a much worse punishment is due the one who has contempt for the Son of God, considers unclean the covenant-blood by which he was consecrated, and insults the spirit of grace? We know the one who said: “Vengeance is mine; I will repay,” and again: “The Lord will judge his people.” It is a fearful thing to fall into the hands of the living God.
Yes, we need sanctifying grace to get us to heaven. It stays with us permanently (unless we lose it by mortal sin). But there is another type of grace — actual graces — which God gives us as an intervention when we need it. If we have lost sanctifying grace, God may give us an actual grace urging us to repent and confess our sins, so that sanctifying grace might be restored.
- We know because Jesus told His apostles, “Receive the holy Spirit. Whose sins you forgive are forgiven them, and whose sins you retain are retained.” (John 20:22–23) This authority has been handed on from the apostles to bishops and priests through the laying on of hands in the sacrament of Holy Orders. When we repent and use the Sacrament of Reconciliation, we are assured of God’s forgiveness and the restoration of grace (the life of God in us). We don’t earn grace back; it is a free, unmerited gift from God. But we can refuse it by serious sin, and God does not force Himself on us.
One more thing — resist the false idea that something must be in the Bible for it to be true. From the Bible itself:
There are also many other things that Jesus did, but if these were to be described individually, I do not think the whole world would contain the books that would be written. (John 21:25)
So obviously Jesus gave the apostles many more instructions than those that were written down in the Bible. The Bible itself tells us we must also believe what is handed down to us by sacred Tradition:
Therefore, brothers, stand firm and hold fast to the traditions that you were taught, either by an oral statement or by a letter of ours. (2 Thessalonians 2:15).
. The Sacrament of Reconciliation restores that grace and washes away the stain of sin; and starts rebuilding the throne 
.