Your mother’s understanding suffers from a common misconception.
This is what the Catechism of the Catholic Church says about suicide:
CCC 2280 Everyone is responsible for his life before God who has given it to him. It is God who remains the sovereign Master of life. We are obliged to accept life gratefully and preserve it for his honor and the salvation of our souls. We are stewards, not owners, of the life God has entrusted to us. It is not ours to dispose of.
CCC 2281 Suicide contradicts the natural inclination of the human being to preserve and perpetuate his life. It is gravely contrary to the just love of self. It likewise offends love of neighbor because it unjustly breaks the ties of solidarity with family, nation, and other human societies to which we continue to have obligations. Suicide is contrary to love for the living God.
CCC 2282 If suicide is committed with the intention of setting an example, especially to the young, it also takes on the gravity of scandal. Voluntary co-operation in suicide is contrary to the moral law.
Grave psychological disturbances, anguish, or grave fear of hardship, suffering, or torture can diminish the responsibility of the one committing suicide.
CCC 2283 We should not despair of the eternal salvation of persons who have taken their own lives. By ways known to him alone, God can provide the opportunity for salutary repentance. The Church prays for persons who have taken their own lives.
In other words, taking your own life is a homicide, just as surely as if you had killed anyone else. It is gravely wrong to choose to do it.
Having said that, however, the Church recognizes that suicide is not always a rational choice. One of the conditions that make something a mortal sin is that it be a deliberate choice, not made under duress or with distorted knowledge. In the case of a suicide victim, this is obviously something that is for God to judge.
No matter how old we get to be, our life on earth is short. We were made for an eternity spent in the love of God, which is total bliss. God in the Second Person emptied himself, became a man, suffered, and was killed in a horrible way, asking that those who killed him receive forgiveness, to save us from our sins. Do not fear God, then. God is obviously on your side. Even when you are beyond loving yourself or wanting happiness or joy for yourself, God still thirsts for that for you. We should only fear that we will be sold on sin, the unreality that makes false promises about what will lead to our happiness. The only thing that will not change to save us is reality. What is false won’t become true in order to give us true happiness, because that is impossible.