But this very way of thinking can end up to one extreme, thinking salvation is all on us by placing so much power in a single sin.
St. Therese and St. Faustina have adequately addressed this in their teachings. Salvation is not “all on us” as we are ALL saved only through the mercy of God. Did you mean “distrust in God’s mercy”? I see no point in distrusting the mercy of our loving Lord Jesus Christ, especially when the point is to move beyond worrying about salvation and have one’s concern about sin be motivated by a genuine desire to love, please, and not hurt/ upset Jesus.
Some people are scrupulous, for example.
Scrupulosity is a mental illness. I feel empathy for those who have it, but many of us are not mentally ill in this way and do not have a big issue taking responsibility for sins, especially big sins, we commit. I don’t have scrupulosity. I am quite capable of distinguishing between a big sin and little sin, and accepting it if my confessor tells me something isn’t really a sin (this has happened a couple times).
And I think sometimes people have bad notions of mortal sin.
If you’re confessing on a regular basis (like I said I go twice a month) and trusting in what your confessor says , not worrying that he was somehow wrong, and not suffering from scrupulosity, this is not a problem because any “bad notion” is quickly corrected.
So we end up misplacing trust in God’s sufficient grace.
Who’s this “we”? I don’t have this problem. Again if you’re talking about people who have the mental illness of scrupulosity, or who don’t go to regular confession, or who just have some Jansenistic hangup, then that’s their hangup and they need to trust God. The writings of St. Therese who is a Doctor of the Church and also struggled with scrupulosity, yet reached the point where she could teach novices how to avoid purgatory, and St. Faustina’s revelations on the Divine Mercy, are most helpful in this area. If one just said St. Faustina’s prayer “Oh Gracious and Merciful God, infinite goodness…(etc)” every day and believed it, one wouldn’t be “misplacing trust in God’s sufficient grace.”
This is where the Protestant is sometimes correct in criticizing the Catholic view that we can easily fall out of grace.
Protestants aren’t correct about anything of the sort. Catholics who think in the way you describe either have scrupulosity, a mental illness, or else they haven’t bothered to catechize themselves sufficiently on God’s mercy and forgiveness and accept the teachings. In any event, this type of thinking is hardly common to all Catholics. Not everyone sits around dwelling on the awfulness of their habitual sins all the time, and many of us work on developing habits of not sinning so we have less to dwell on and worry about.
To reject the mercy of God when you don’t have the excuse of being mentally unwell is rather prideful in my book, as well as silly. Why would anyone who believes in an all-powerful and loving God do that? No one’s sins are that extra special.