I think it is important here ot compare the pedantic differences between what constitutes
Grave Matter and what constitutes a
Mortal Sin.
The church defines a very wide range of things as being Grave matter.
For a Grave Act to be a mortal sin there needs to be several other conditions as well as just the act.
Knowledge that it is Grave Matter (Potentially a Mortal Sin)
Full ascent of the Will (It is not a mortal sin if your choice is not fully free and deliberate)
Sufficient time for reflection (A snap decision later regretted is unlikely to be a mortal sin)
Ultimately a mortal sin occurs when you choose to carry out a deed, knowing it could send you to hell, having time to consider the implications, and going ahead and doing it anyway.
(a bit like the difference between a premeditated, cold-blooded “Murder 1”, knowing you’d be caught, compared to a lesser crime like “Voluntary Manslaughter” or even “Justifiable homicide” )
In each case the act is the same - you killed someone - but the level of guilt and the “punishment” are very different.
When you come face to face with God after committing an act of premeditated deliberate mortal sin, knowing God was watching you, and knowing that that act would seriously damage your relationship with Him - How if you came face to face with Him at that moment could you look him in the face and say (honestly) “I Love You, I’m sorry, Please Forgive Me!”. How could you enter his embrace? How could you accept his forgiveness?
In that state you would be almost certain to refuse his offered forgiveness and cast yourself into the Pit, from Pride or Shame. - That is the effect of Mortal Sin. That is the difference between Mortal Sin, and other sins that also stem from Grave Matter. - How much damage it does to your ability to accept the Love and Forgiveness of God.
- But all sin is Forgivable - you just need to repent of it. You need to say Sorry - and mean it.
The normal way of saying “Sorry” is in the confessional - but it is not the only way, and God’s mercy is not restricted to our human disciplines.
However when alive and healthy and there is no grave reason to disregard them, to disobey those disciplines can in itself be a Grave Matter. - It is a rejection of the Church to which Christ gave the Power to Bind and Loose. - It is therefore a Rejection of the Body of Christ (In the form of the Church, not the Eucharist).
To receive the Eucharist in this state is in itself a grave sin - (in a similar way If you were having a serious argument with your wife, to insist on having sex right then - without would be Raping her. It would be a very grave sin against her .) - It would be using an act… a gift of Christ… which is meant to bring us closer together at a time and in a way that is totally incompatible with that unintuitive function - thereby causing further significant harm.
It is vital to “Make up” after the argument - to say sorry… then the unitive function of that act (receiving the Eucharist or in the analogy - sharing the marital embrace) can be expressed and serve it’s proper function.