Sorry I didn't get back to you sooner. I had to do a little research to get a correct answer to your question.
I have this in a few sources but the clearest is from the Catechism of Pius X.
Below I have copied and pasted the relevant section from an internet source. Notice in particular numbers 84 and 85 which clearly confirm that [he/she] must redo all of the Confession not just the part [he/she] left out.
| The Accusation of Sins to the Confessor |
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| Question 71. |
Having prepared properly for Confession by an examination of conscience, by exciting sorrow, and by forming a good resolution, what do you do next? |
| Answer: |
Having prepared properly for Confession by an examination of conscience, by sorrow, and by a purpose of amendment, I will go to make an accusation of my sins to the Confessor in order to get absolution. |
| Question 72. |
What sins are we bound to confess? |
| Answer: |
We are bound to confess all our mortal sins; it is well, however, to confess our venial sins also. |
| Question 73. |
Which are the qualities the accusation of sins, or Confession, ought to have? |
| Answer: |
The principal qualities which the accusation of our sins ought to have are five:
- It ought to be humble
- entire
- sincere
- prudent, and
- brief.
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| Question 74. |
What is meant by saying that the accusation ought to be humble? |
| Answer: |
That the accusation ought to be humble, means that the penitent should accuse himself to his Confessor without pride or boasting; but with the feelings of one who is guilty, who confesses his guilt, and who appears before his judge. |
| Question 75. |
What is meant by saying that the accusation ought to be entire? |
| Answer: |
That the accusation ought to be entire means that all mortal sins we are conscious of having committed since our last good Confession must be made known, together with the circumstances and number. |
| Question 76. |
What circumstances must be made known for the accusation to be entire? |
| Answer: |
For the accusation to be entire, the circumstances which change the species of the sin must be made known. |
| Question 77. |
Which are the circumstances which change the species of a sin? |
| Answer: |
The circumstances which change the species of a sin are:
- Those by which a sinful action from being venial becomes mortal, and
- Those by means of which a sinful action contains the malice of two or more mortal sins.
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| Question 78. |
Give an example of a circumstance making a venial sin mortal. |
| Answer: |
If, to excuse himself, a man were to tell a lie and by doing so occasion serious harm to another, he would be bound to make known this circumstance, which changes the lie from an officious lie to a seriously harmful lie. |
| Question 79. |
Give an example of a circumstance on account of which a single sinful action contains the malice of two or more sins. |
| Answer: |
If a man were to steal a sacred object he would be bound to accuse himself of this circumstance which adds to the theft the malice of sacrilege. |
| Question 80. |
If a penitent is not certain of having committed a sin must he confess it? |
| Answer: |
If a penitent is not certain of having committed a sin he is not bound to confess it; and if he does confess it, he should add that he is not certain of having committed it. |
| Question 81. |
What should he do who does not remember the exact number of his sins? |
| Answer |
He who does not distinctly remember the number of his sins must mention the number as nearly as he can. |
| Question 82. |
Does he who through forgetfulness does not confess a mortal sin, or a necessary circumstance, make a good Confession? |
| Answer: |
He who through pure forgetfulness does not confess a mortal sin, or a necessary circumstance, makes a good Confession, provided he has been duly diligent in trying to remember it. |
| Question 83. |
If a mortal sin, forgotten in Confession, is afterwards remembered, are we bound to confess it in another Confession? |
| Answer: |
If a mortal sin forgotten in Confession is afterwards remembered we are certainly bound to confess it the next time we go to Confession. |
| Question 84. |
What does he commit who, through shame or some other motive, willfully conceals a mortal sin in Confession? |
| Answer: |
He who, through shame or some other motive, willfully conceals a mortal sin in Confession, profanes the sacrament and is consequently guilty of a very great sacrilege. |
| Question 85. |
In what way must he relieve his conscience who has willfully concealed a mortal sin in Confession? |
| Answer: |
He who has willfully concealed a mortal sin in Confession, must:
- reveal to his Confessor the sin concealed
- say in how many Confessions he has concealed it, and
- make all these Confessions over again, from the last good Confession. [top]
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| Question 86. |
What reflection should a penitent make who is tempted to conceal a sin in Confession? |
| Answer: |
He who is tempted to conceal a mortal sin in Confession should reflect:
- That he was not ashamed to sin, in the presence of God who sees all.
- That it is better to manifest his sin secretly to the Confessor than to live tormented by sin, die an unhappy death, and be covered with shame before the whole world on the day of general judgment.
- That the Confessor is bound by the seal of Confession under the gravest sin and under threat of the severest punishments both temporal and eternal.
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| Question 87. |
What is meant by saying that the accusation ought to be sincere? |
| Answer: |
By saying that the accusation ought to be sincere, is meant that we must unfold our sins as they are, without excusing them, lessening them, or increasing them. |
| Question 88. |
What is meant by saying that the Confession ought to be prudent? |
| Answer |
That the Confession ought to be prudent, means that in confessing our sins we should use the most careful words possible and be on our guard against revealing the sins of others. |
| Question 89. |
What is meant by saying the Confession ought to be short? |
| Answer: |
That the Confession ought to be short, means that we should say nothing that is useless for the purpose of Confession. |
| Question 90. |
Is it not a heavy burden to be obliged to confess one's sins to another, especially when these are shameful sins? |
| Answer: |
Although it may be a heavy burden to confess one's sins to another, still it must be done, because it is of divine precept, and because pardon can be obtained in no other way; and, moreover, because the difficulty is compensated by many advantages and great consolations. |
I'm sure the Confessor will be able to assist you, in what is needed, the next time you go to Confession.
Hope this helps.
Fr. Jonathan
