The last "You have heard it said" in Matthew 5 is: 
        Matthew 5:43, "You have heard that it has been said, You shall love your neighbor, and hate your enemy."
       And the last "But I say unto you," is an indictment for all of us.  Matthew 5:44-46, "But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you and persecute you..."  I bet everyone in the crowd that day was shocked.  They had never in their lives heard anyone say something like that.  Just think:
1.  Love your enemies
2.  Bless them that curse you
3.  Do good to them that hate you
4.  Pray for those who despitefully use you
5.  Pray for those who persecute you
        Isn't it interesting that we take some of Jesus words to heart, and others--like the five instructions Jesus gave the crowd that day--we tend to skip over.  We don't like any of those things He said.
       And the next sentence out of Jesus' mouth was:  "That you may be the children of your Father which is in heaven...For if you love them which love you, what reward have you?  Don't even the publicans do the same?"  What He said is that doing those things shows that we are God's children.

The law was impossible to keep.  But Jesus was telling them--and us--that there is an even higher law:  The law that God writes on our hearts.  Something we carry around with us all the time.

Ezekiel 36:26 "A new heart also will I give you, and a new spirit will I put within you: and I will take away the stony heart out of your flesh, and I will give you an heart of flesh."
When we accept Jesus as God and Savior, he changes our hearts.  Otherwise it would be hopeless.