I understand that there needs to be three conditions met at the same time for the sin to be mortal, but the condition of "full consent" is a very legalistic term. Example: What is full consent of the will for me, may be different to you due to psychological, habitual, duress, etc disorders, same with proper knowledge, but this is in regards to "consent only"
Since it is impossible for us to know if we are consenting 100%, this leaves us in the state of pondering if we did so 100% or not, so since we are left pondering this means that we obviously did not know for certain, which makes the sin venial. (Remember: we must need FULL CONSENT of the will.
A legalistic statement leads to a legalistic answer, here is another example:
Joe Catholic has had a habit of being "impure" with himself for 25 years, he also suffers from certain psychological disorders...anxiety, panic, and depression. He falls into the act of being "impure" with him self. The Catholic Church says this is a mortal sin if ALL 3 of the classifications of mortal sin are met, but Joe is uncertain if he consented 100%, he thinks he may have, or may not...simply, Joe is uncertain which takes away the condition of full consent. He still ask God to forgive him and give him strength to overcome the desire and action, but it is NOT a mortal sin.
What I'm saying simply is this..we need not to get so worked up over things we do not know if we are completely guilty of, are we still sinning ? to an extent yes! mortally? no!
I personally find confession to be awesome, but when people startgetting so legalistic over sins it becomes a problem..just remember what St. Paul said: Romans 7:15-20
I do not understand what I do. For what I want to do I do not do, but what I hate I do. 16 And if I do what I do not want to do, I agree that the law is good. 17 As it is, it is no longer I myself who do it, but it is sin living in me. 18 For I know that good itself does not dwell in me, that is, in my sinful nature.[c] For I have the desire to do what is good, but I cannot carry it out. 19 For I do not do the good I want to do, but the evil I do not want to do—this I keep on doing. 20 Now if I do what I do not want to do, it is no longer I who do it, but it is sin living in me that does it.