A sin becomes mortal once the person knows or believes it is grave matter and fully consents to do it anyway.
For example, if I was going to watch pornography but found out my VCR was broken, I would still have committed a mortal sin since I fully intended to watch pornography. If I started watching poronography then said to myself “What I am doing?” and realised the evil I was committing and shut it off, I would still have sinned mortally. If I intended to kill someone and when I saw them I thought to myself “I can’t go through with this” I would still have sinned mortally since I intended to kill that person. Of course, going through with killing that person would be an even greater sin.
Don’t figure just because you committed one mortal sin you should keep on committing them because you already deprived yourself of communion with God. For example, if you screwed up and viewed a pornographic image, don’t say to yourself “Well, I screwed up so I might as well view more pornography and then confess it all at Reconciliation.” This is what the devil wants you to do and also it is the sin of presumtion. It also indicates a lack of contrtion for the first mortal sin you committed.