Jesus fulfilled the Law by living it. The law is summed up he taught as “You shall love your neighbor as yourself.”
The first commandment is to love God with the entirety of our being; the second “is like it,” which is to love our neighbours as we love ourselves. You cannot seperate the two and, if anything, the emphasis is on the first.
He did not fulfill it by prescribed rites.
He most certainly did. He embodied all of those rites perfectly. That is why it is said that all those rites pointed to and prefigured Him. Jesus is the Lamb of God. The liturgical context of Our Lord’s Passion (the Passover) is enormously significant for demonstrating that He did fulfil those rites in His very Person.
He called us to fulfill it as He did but we do not. None of us.
In one sense, only Our Lord could fulfill the whole Law, if in that we include also the rites, etc.; after all, they did prefigure Him. In another sense, however, all Christians are to keep His commandments (the Law) and we certainly are able to do so; otherwise, we make Christ into a tyrant Who commands the impossible. Even secular (common) law recognises that the law cannot enforce an impossibility.
What He taught is that God loves us nevertheless and forgives us and we should live in that light.
This would make His enjoining us to repentance pointless. Even after He accomplished the Sacrifice of the Cross, He still commanded His holy Apostles to go forth into the world preaching repentance of sins; however, if God had forgiven all men their sins
ipso facto, as it were, then repentance of them would be mute and pointless.
He did not fulfill the law by changing it. He explained it as summed up in Gods love and our love for our neighbor.
Again, the commandment is for us to i) love Him and secondly ii) our neighbours as ourselves. Both of these are positive commandments. We are enjoined to actually and actively love God and also our neighbours. Neglect of this is to violate His holy commandments. This is part of the reason why Christians retained the Ten Commandments and enjoined them.
So we trust in the promises of Jesus rather than our own ability to meet the Law.
We are absolutely capable of keeping the law with God’s help. God would not enjoin what is impossible upon us. His promises assure us of His help. An act of Faith consists in trusting that God will not foresake us and that He will grant what we need to keep His Commandments; through His mercy, He has given us the Sacrament of Confession in case we fall (break the commandments); nonetheless, that is a proof of His mercifulness and not a license to break His commandments.
For example:
You may have a general inclination to forgive your children their faults or trespasses but you would become angry with them if they abused your love and mercy so as to grant themselves a license to break your commands, rules, precepts, etc. When you chastise your children generally you search them out for interior signs of sincere repentance or sorrow; and once detected, you then comfort and console them because you know such sorrow proves a change of heart; however, if you see them rolling their eyes or only producing words to distract you, then you know they haven’t learned their lesson, and may proceed to punish them (e.g. a grounding); whereas, had they fallen in tears begging your pardon, you may have been inclined to forgive them on the spot. Of course, you do not enjoin upon your children things impossible for them to actually keep: you know their capacities and limitations and you know what is easier or more difficult for them to keep. Because you are rational, you can judge prudently how much rigidity or laxity should be given in the maintence of your rule, as it were.