I don’t really agree with the Catholic understanding of mortal and venial sins and the categories of sins and the listing that can derive from it. I think it results from first turning the ransom metaphor into a legal Roman court metaphor and clouding the imagery given through the ransom metaphor in the NT and elsewhere and then taking that analogy which a referent to concrete realities (itself already a little misunderstood with a different metaphor then intended) with the concrete reality itself (understanding of course within Roman Catholic discourse and thought it gets much more complex and nuanced then this simplistic presentation but I think this is still leads to some deeper misunderstandings and problems, such as the above were it is presented without further elaboration). Of course the same misunderstanding is present it much of Protestantism just with different ways of working out but leads I think to it’s own problems though this is changing in a number of places. (and within Roman Catholicism itself I think).
However with that preface if my views the following link from a Roman Catholic theologian and persist might help as it indicates that it isn’t in this view a mortal sin to miss Mass while still being faithful ad a Catholic but only venial. Only if you abandoned or turned from it or the Church or Christ altogether. Anyway give it a read Kate and I hope it brings some relief, also remember those Catholic rules ae have discussed in our pms and don’t feel guilt for confessed sin or let it or listen to it telling you are guilty when it is gone (if you do feel it is a sin in the first place, if you aren’t sure then act in confidence it is not) and don’t let the thoughts bother you again .