Jesus said in Matthew 5:17-20,
"Do not think that I have come to do away with, undo, or abolish the Law or the Prophets; I have come not to do away with, undo, or abolish but to complete and fulfill them. For truly I tell you, until heaven and earth disappear, not the smallest letter, not the least stroke of a pen, will by any means disappear from the Law until all things are accomplished.
Therefore anyone who sets aside one of the least of these commands and teaches others accordingly will be called least in the kingdom of heaven, but whoever practices and teaches these commands will be called great in the kingdom of heaven. For I tell you that unless your righteousness surpasses that of the Pharisees and the teachers of the law, you will certainly not enter the kingdom of heaven."
Jesus saying He didn't come to do away with the law but to fulfill it is like Him saying, "I didn't come to do away with the debt. I came to pay the debt." Paying the debt is like doing away with it but better because it is more than mercy, it is forgiveness and graciousness. We were in debt to pay the whole law with perfect obedience to it. Jesus did it on our behalf. He paid the debt of obedience. Jesus wasn't saying, "Forget about that old law thing I'm going to junk it." He was saying, "I'm going to do this for you because I love you and I know you can't do it."
If Jesus was speaking about these things after His resurrection He would say something like this.
"When I came I didn't throw out the law and the prophets. I have fulfilled the requirements of the law for all mankind. I have also fulfilled the prophesies about the Messiah. What I did was so important that even if heaven and earth disappeared every bit of the law would have remained unfulfilled but I accomplished all things necessary to fulfill it. Now I make everyone great in my kingdom. Even those who were the best law keepers did not have enough righteousness to enter my kingdom. You will need my righteousness to enter my kingdom. Just start trusting me and I will give it to you."