If you want a simple explanation of mortal sin, the CC teaches that there are a number of requirements for any sin to be mortal, and that it must be, in itself, a grave matter. They include full knowledge of what is being done and its implications, full control over oneself, and intent to do the bad thing.
This being the case, it is impossible to say that any sin, in general, is either mortal or venial. Sins do not exist as a category or an abstraction, they only exist as particular incidents in particular individuals. In any given example, the above criteria may or may not be met.
To say that any example of breaking a commandment is a mortal sin is, on the face of it, totally ridiculous. Do you really think that any time a 10 year old is disrespectful to his parents he is in mortal sin? Even sins like murder are not always mortal sins.
And on the other side, what are normally considered venial sins can surely separate people from God as much as any more outwardly impressive sin can. You suggested anger as only a venial sin - that is what in the old days used to be called wrath - one of the seven deadly sins. Anger, even fairly interior and unexpressed anger, which we allow to take root in our hearts, can be an extremely sure way to cut us off from life in Christ.
If you want a more complicated answer, I'd suggest going beyond the CCC and actually reading some of the saints and what they said about the nature of sin - Augustine who I have already mentioned in this thread is a good place to start. You might also find it useful to do some reading on theosis.
What this means is that you can't say "missing mass is a mortal sin" unless you know the specific circumstances, and then it is still an educated guess, at best. You could say that missing mass is, or can be, a grave matter, and as such it could possibly constitute a mortal sin.