Hi Vinny,
Yes one can fulfill the law, properly understood as it ties to agape and participating in the divine nature. It is kept in a qualitative, not quantitative, sense. One can grow further in agape, but as agape is participation in the divine nature, it is not somehow “imperfect” because of this (similar to st therese’s analogy of saints’ glorification in heaven where a “lesser” saint has a thimble-full of water and a “greater” saint has a gallon of water - both are perfectly full for each person).
A sharp Catholic at the website Called to Communion website described this as the “list vs agape paradigm” - the list paradigm being a set of rules (like in the OT) one must keep which is how your questions seem to be framed. I would suggest reading
this link and its comments for an extensive discussion.
Also, related to this is how we reconcile biblical and Augustinian statements you’ve cited indicating we all sin and need forgiveness, with the view that we can fulfill the law in some sense. I would say we would also have to reconcile those statements with other biblical statements like:
“love is the fulfilling of the law”
“Bear one another’s burdens, and so fulfill the law of Christ.”
“If, however, you are fulfilling the royal law, according to the Scripture, “You shall love your neighbor as yourself,” you are doing well.”
“By this we know that we love the children of God, when we love God and observe His commandments. For this is the love of God, that we keep His commandments; and His commandments are not burdensome.”
“The man who says, ‘I know him,’ but does not do what he commands is a liar, and the truth is not in him. But if anyone obeys his word, God’s love is truly made complete in him. This is how we know we are in him: whoever claims to live in him must walk as Jesus did”
“If you keep My commandments, you will abide in My love, just as I have kept My Father’s commandments and abide in His love”
So we know we can keep the law in one sense, but yet fail to be “perfect” (Catholicism does not teach Wesleyan perfectionism). This naturally leads to the distinction between venial and mortal sin. Mortal sin destroys agape, venial does not. Another CTC article focuses just on that (and ties into the agape/list paradigms) which you can read
here
Thirdly because this started off with questions on merit - there is a link on CTC to a great audio discussion by a professor focused on merit you can check out
here
Hope this helps.