In terms of confession of old mortal sins that one is doubtful that one confessed them or committed them…normally one would be advised to confess them…and if they were truly forgotten mortal sins one would normally be obliged to confess them.
But a person who is “very scrupulous” can be in a very different boat. They ought to have a regular confessor to direct them.
A few first points then more advice:
One must distinguish between a scrupulous conscience and a delicate conscience.
A ‘tender conscience or delicate conscience’ sees the finer distinctions and seeks very much to avoid even very small sins …and …who can judge normally between mortal and venial sin…when in doubt is able to make judgments overcoming his doubt by prudence or abide by good advice etc
as for scruples…or scrupulosity
scruples or those who are even of a higher degree of scrupulosity take venial sin to be mortal…or non sins to be sins…or are plagued by doubts…unreasonable fears/anxieties about sins…seeing them or fearing them where they need not…etc etc
The french call it the doubting disease (though it is not a disease…and I would use the term “mental illness” either…but it is certainly a disorder within the person) for one has lot of problems knowing one knows…thus with doubt (or more often “apparent doubt”)
of course there are degrees and it is possible for a person to have generally a delicate conscience or a normal conscience and still have a struggle with scruples in certain areas…or have very transient scruples at the time of their conversion …
and a person may go from one to the other…
one combats scruples with a good
regular confessor…
by forming ones conscience…(with such as the Catechism etc)
and with his guidance acting against ones scruples…
possibly by general principles given often historically by confessors to deal with scruples
by prayer…
by trust and joy in living the Christian life…(not necessarily “emotional” by the way)
With the help of your confessor…etc
Contra scruplos agendum est, et fixo operis pede certandum
(Act contrary to scruples and with a firm foot overcome them)
Overcoming ones scruples…ones fears with a “firm foot”. Learning to act against them under the guidance of your confessor. Dismiss them. Not argue with them. (ie. ones confessor says…“unless you are certain you have committed a mortal sin…go to communion”…and you feel the fear and the doubts and the worries at the time of communion “but perhaps I committed one…?” you tell yourself “I am not certain of any mortal sins and my confessor told me to go” and you act against your fears and go to Communion! or ones confessor says regarding old past sins “only confess old sins if you are certain it was a mortal sin and certain you have not confessed it…(such is often the advice for those who are scrupulous)” one does just that -and if one is mistaken here one it not on you for a it can be the case that those who are scrupulous are in a “different boat” that others.) etc
(in a way one needs be “scrupulous” about attending to ones scruples…)
When scruples arise --turn to God and let the pass by.
Scruples are to be treated like a barking dog or a hissing goose…one does not stop to argue with a barking dog or a hissing goose does one? (image borrowed from a 15th cent. Carthusian).
They are to be “despised”…“dismissed”…
And again a regular confessor is very important here…in even coming to know what ones scruples are…and then what to dismiss…
Hope that helps. Feel free to pm if I can be of other help.
This thread would be fine…though…the word confession might not be the best title for a scrupulous thread. 
Prayers for Lois (his grandma) are most appreciated and welcome. 
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