Jesus tells us he didn't come to "abolish the law and the prophets," but "to fulfill them." What's he getting at here?

I think he is preparing us for what he's going to say next. Jesus is not against the Mosaic law or the Ten Commandments. He is asking for so much more. He's been striking out at the religious leaders and how they treat those laws and how they enforce them. He is ultimately asking for more from them and us. They have been without compassion and mercy. The very next line after this passage is:

"I tell you, unless your righteousness surpasses that of the scribes and Pharisees, you will not enter into the kingdom of heaven." Matthew 5:20.

The scribes and Pharisees obeyed the letter of the law; how can their holiness be lacking? It is because they lack mercy. They lack the ability to surrender their judgments of others. Holiness is not so much about obedience to the law as it is about having the heart that Jesus has.

In the rest of this chapter, he will be telling us, "You have heard it said," quoting from the law, and then he will say, "but I say to you ..."

The law says not to kill. Jesus tells us not to be angry at one another.
The law says not to commit adultery. Jesus says that to look at a woman with lust is to commit adultery of the heart.
The law says "an eye for an eye, a tooth for a tooth." Jesus says we should offer no resistance to evil.
The law says "love your neighbor and hate your enemy." Jesus wants us to love our enemies and to pray for those who persecute us.

It is a great mid-Lent gospel for us. However our Lenten journey has gone so far, we can ask ourselves how we are doing in letting our lives be shaped by the heart of Jesus. And, when we discover any remaining resistance to being changed, we can ask for the grace to spend time this week letting ourselves be reminded that this is the way Jesus loves us. He doesn't give up on us. He doesn't judge us. He knows us through and through, being with us every day, in every moment of our lives. And, he loves us. He doesn't love us because we've made a great Lent so far. He loves us because he knows our struggle and his heart goes out to us. He wants to help us. He desires to heal us.

We cooperate with that grace - here in the middle of Lent - by asking, "Lord, you know what needs healing in me. Help me name it today and let me ask you to come in and work your healing in me. Meet me there, in that place, dear Lord." Trusting in his grace, we can "practice" being more attentive to someone today. We can "practice" a newly desired kindness. We can "practice" complimenting, affirming, thanking someone today. Taking steps each day toward thinking of the needs of another close to me, and acting out of love, will help me be more and more open to thinking about the needs of those most in need in our world.

It is so good there are still several weeks of this blessed time of Lent before us.