True, this is in the Catechism:
z_0101;12837837:
Under CCC1459 and 1491: (…);
Glad we can agree on that.
This is not in the Catechism and is z’s opinion:
z_0101;12837837:
however, to add an additional penance outside of that prescribed by the priest is not in line with what I understand
…
I thought I was very clear that it was/is my opinion in the post - no?
With that said, the opinion is based upon sound teachings. , From both NT and OT that to add to God’s will can be sinful and as illustrated by Moses’ act. Keep in mind, that just because something isn’t clearly stated within Church teaching doesn’t mean we can go and do as we please - Moses did…

.
BTW: We have addressed this very thing in our RCIA classes. Father was very clear that we should trust the Celebrant of the Sacrament - if no penance is given, then we are to make reparations to those harmed if possible and trust in the mercy of God and that we have received absolution. Thus, this is not solely my opinion but also based upon a sound catecheses program.
It is meritorious and completely in line with the tradition of the Church to do extra penance as one recognizes one’s sinfulness before God.
With in the context of the sacrament.
The obligation to do penance can be fulfilled by the act of reparation - say in the case of Gossip… it would take a great act of courage to speak up to correct the misinformation/lies told. This is within the norms of the Church magisterium.
However, they should run it by their spiritual director if it is a fairly rigorous penance to*** avoid scruples or spiritual harm***
That is the exact reason one should not be adding penance to that given by the confessor - scruples **and **spiritual harm. Doing anything more than the penance given and making reasonable and appropriate restitution to those harmed is outside of the norms
To the OP’s original question: If you forgot your penance
OP didn’t forget the penance. OP was not assigned a penance that OP was aware of; thus, either the reparations due to those hurt was sufficient in the Celebrant’s opinion or the Celebrant did the penance for OP. In either case OP should trust the Confessor’s act and have faith in God’s mercy.
or if the priest forgot to assign it to you: go ahead and substitute a good work or prayer for your penance.
It is not in the norms of either CCC nor the CL for the penitent to assign their own penance.
Can. 981 The** confessor is** to impose salutary and appropriate penances, in proportion to the kind and number of sins confessed,** taking into account, however, the condition of the penitent.** The penitent is bound personally to fulfil these penances.
That is really very clear… the confessor,
not the penitent, is to impose the penance. In the case of OP, we do not know the condition of OP at the time of confession. It could be that the Celebrant felt that additional penance would do more harm than good. We were not there (nor should we be

).
I did and it doesn’t change the fact that the penitent is to trust in the Celebrant, do only the penance(s) imposed and to make appropriate recompense to those damaged by the penitent’s act when such is possible in accord with the CCC and CL!
Let’s bring in that forgot penance you mention… From the above link:
People will, once in a while, forget the penance that was assigned. This can happen when some well-meaning priest assigns one of those loopy, long, open-ended penances, such as, “Read the seventh chapter of the Second Book of Kings and spend some time under an elm tree counting ladybugs while you think about the impact your use of fossil fuels has on the environment.”
In those cases, just substituted some good work, as you seem to have done.
Going to confession the next time you could say that you didn’t do an assigned penance because you didn’t understand it, forget it, couldn’t do it… whatever… and that you, instead, substituted 10 Our Fathers and 10 Hail Marys.
In the meantime, rest confident in the fact that your sins were forgiven. You can receive Communion without any qualms on that point of the penance.
Here, one is attempting to act in good faith to do penance and then brings the sin of not completing the imposed penance back to the confessional for absolution along with a mitigating act which the Celebrant can take into consideration when imposing penance.
And while we’re reading FrZ’s blog… let’s get the bigger picture from his point of view:
Am I forgiven if I don’t do a penance assigned in confession? Fr. Z rants. (it’s a long article, please follow the link for full context)
In any event, the penances assigned in confession **do not affect the efficacy **of the sacramental absolution we receive.