Q. I am burdened with the need to engage in some serious restitution. I have made a list of all the people that I have cheated, and it is extensive. I do not know where to start.
A. Unless you engaged in a deliberate and sustained process of defrauding people of tangible property, fully aware of the choices you were making, restitution is not required. You are not required to make restitution for normal, imperfect, and human misuse of time (the concern most expressed by the scrupulous) or anything else. Restitution is not your issue. I surmise that you are having difficulty with remorse and the inability to forgive yourself for being human.
Q. When does a venial sin become a mortal sin?
A. Never. Venial sins do not add up to a certain number and then morph into a mortal sin. A venial sin is not a serious matter; a mortal sin is serious. One hundred sins that are not serious, for example, do not add up to one mortal sin. There is no accumulating value to a venial sin. A venial sin stays a venial sin, nothing more.