Mama ChaCha,
It is basic Catholic teaching that in order for a person to commit a moral sin, he must do so with full knowledge (both of the sin and its gravity) and he must do so with deliberate and complete consent.
In other words, you cannot accidentally commit a mortal sin.
Additionally, even if you did commit a mortal sin, self-imposed penance can be a dangerous practice. It would be better simply to go to confession and do what penance the priest directs, and leave it at that.