Might I ask for source that ‘grave sin and mortal sin are not the same thing?’
I am sure that at some point in the last 60 or so years of chaos and ‘teachings’ that some people, even theologians, may have ‘mentioned’ this so-called ‘grave sin’ and put it in ‘opposition’ to ‘mortal sin’.
But a few theologians opinions do not a Church teaching make.
There is no difference between ‘grave sin’ and ‘mortal sin’.
There is only a difference between objective mortal sin (which is what both grave and mortal sin are) for which a person has full culpability (i.e., sufficient knowledge and full consent) or lessened culpability. Lessened culpability is NOT, repeat NOT, ‘grave sin that is not mortal’. It is grave sin that is mortal but the person MAY not have been able to freely consent or have truly had knowledge. Once however the person has knowledge one may no longer claim innocence through ignorance. And with regard to full consent, even in the face of danger and death one is not so coerced that one may not choose the right way.
So yes, I believe you are wrong. I had the Baltimore catechism as well. TWO sins, mortal and venial, not THREE sins, mortal, grave, and venial.