The requirements for a mortal sin are as follows:
1857 For a sin to be mortal, three conditions must together be met: “Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent.”
Now, if you are committing a sin with full knowledge that it is a sin, is that not like thumbing your nose at God? And as such, would that not be a grave matter in itself? Eating an apple seems innocent enough, but God commanded Eve not to do it, and she did it anyway, and all of humanity fell because of it.
well, as it says,
“Mortal sin is sin whose object is grave matter”,
but the question is, what sins are actually being committed in your example?
eating from the tree of knowledge in and of itself was not the sin God was offended with, He did not love the tree, and the tree could not be hurt,
God was rather offended with the disobedience, and the pride(among other things)
the commandments they broke, although they were not established at that time(although maybe God taught them, who knows) were the 1st and 4th commandments,
the 1st because they chose themselves over God, and the 4th because they disobeyed God who is the supreme authority,
so Adam and Eve really did commit mortal sins and not venial, especially since they knew better what they were doing them most of us would,
but as for venial sins, when we commit venial sins we usually don’t know full well what we are doing, the reason being that venial sins are more of a disease to our souls, it is a sickness, it’s something we very often do out of habit or weakness or ignorance,
of course as we know, sicknesses that are not treated can very well lead to death,
and so venial sins not treated and not corrected can lead to mortal sins.
but the reason God is so patient with as, as opposed to how He dealt with the angels, is because we are learning creatures, we go at a slow pace, whereas the angels had infused knowledge, they did not get a second chance because they knew what they were doing and chose without any ignorance, they chose period, whereas we choose more often not knowing fully what we are doing and how we are offending God and hurting others others, we sin very often in ignorance and blindness,
and God understands, especially when he knows we do not want to offend Him or hurt others, many of us have good hearts, but weak bodies, and God understands.
it’s not all black and white of course, but one thing that separates mortal from venial sins is the effect it has, on ourselves, on others,
and also the intention makes a big difference,
like for example, when we hurt someone, but we did not mean to, that is an accident, and isn’t it so much easier to forgive someone you love when you know they did not mean to do what they did?
that is very much like our venial sins, provided we don’t want to commit them, when we do commit them we might say “I am sorry Lord, i did not mean to do that”
and God understands, He will forgive you, although the stain remains, which can be wiped clean by several means, but i won’t get into that right now.
but as for knowing the sins you are committing and not caring? that is a sin in and of itself, and God is more displeased with your lack of caring then He is with whatever sins you are committing,
and i suppose when you care that little, it doesn’t particularly matter what sin you are committing, because you are separated from God nonetheless,
of course it matters when we sin, whether we care or not, but what i mean is, when you sin so carelessly, then the problem lies not in correcting your sins, but in healing and softening your hardened heart, which is careless by lack of love for God, and therein lies the problem,
and of course God truly loves us, He loves the most hardened hearts, and longs to help them more then any others, and because of the loving sacrifice of Our Lord Jesus Christ, God can love us, as great as our sins may be, more then He is displeased with our sins, and so we have great hope in His mercy.(sorry that went slightly off subject)
well, i think i said enough, i hope this helps,
take care.