You are sounding very bitter, I know that is presumptuous of me to say, but there it is. It seems to me you are starting to see what the CC defines as mortal sin as a kind of arbitrary designation. While that seems a common complaint among Catholics, I don’t think that it is really how Catholics are supposed to understand and practice it - I do know some very pious and learned Catholics that would not agree with that approach, including priests.
When we think about sin we need, among other things, to look at the overall pattern of our life. In general, do you seem to be drawing nearer to God, improving at your spiritual and other disciplines? Are you having trouble in general? Are there patters - you get tired and things fall apart? How is your prayer life?
Our relationship to God is about acts as well as attitude, but it is not about a list of “Sin X so many times” defines your relationship with God. It is more like the imagery of Jacob’s ladder, where we go a little way forward now, and back the next day, but we want always, with God’s grace and the disciplines and sacraments of the Church, to trend upward. Sometimes something, or an accumulation of somethings, may block your path, or you may even start to head down in a more active way, and then you need to do something about that.
So in relation to this issue I would be asking myself - is this an act or part of a pattern that is putting a serious block between myself and God? Did I, fully consciously, say no to something essential about God? In not attending was I deliberately saying that I didn’t want to be part of the Christian community or partake of the Eucharist? Do I do this a lot? Was I unusually tired or run down?
Do you have a confessor you trust and see every time, and who knows you and the sins you tend to struggle with, and don’t struggle with? Having someone else who can look at you objectively can go a long way toward helping with issues you are unsure about.