If someone is unsure about whether a sin is mortal or not, doesn’t that mean it isn’t mortal sin?
Saying that mortal sin requires “full” knowledge and consent is a handy memory cue that may useful in catechesis, but it not a precise formulation of the kind one would get in a moral theology manual.
The kind of consent needed, more technically, is the degree of consent needed to make a fully human act. This does not mean saying, “Yes! I want to do something really evil!” and having no reticence about it. One can have misgivings, regrets, mixed feelings, et cetera and still give deliberate consent to an action.
Similarly, one does not have to know with metaphysical certitude that a given act is gravely sinful. Lesser degrees of knowledge will also count—again, the degree of knowledge needed for an authentically human act is the key. Indeed, someone might even be feigning ignorance or being hardhearted toward the evidence in such a way that he is responsible for knowing something even though he professes not to know it.
These facts mean that it is possible to have some doubt—especially after the fact, when one can’t remember one’s precise state of mind—as to whether a given act was mortally sinful. The rule of thumb is, “If you didn’t think it was a mortal sin at the time, it probably wasn’t.” But this is not an invariable rule. There are gray cases.
When presented with a gray case, the received rule of thumb is for persons of normal or lax conscience to go ahead and confess it anyway, to be safe and to have peace of mind.