Today's Gospel reading comes from the Sermon on the Mount and so is formed of short sayings of Jesus which have been strung together according to a seeming similarity of subject.  Jesus here begins by speaking about his fulfilling or completing the Law and the Prophets and then switches to underlining our need to obey the Law completely.  His last words state that incomplete observance of the Law, or even teaching an observance of the Law which is not literal, is still enough to gain one entrance to the Kingdom.  None of this is an obvious part of usual lenten themes. 

A more complete commentary on this passage would discuss how Jesus is indeed the fulfillment of the entire Old Testament and how that fulfillment makes possible a new relationship between God and his people, a relationship of love and heartfelt service that goes beyond a childish and scrupulous observance of the written law.  And such a commentary would develop at some length the fact that any response we might make to God, either in the forms prescribed by the Law or in the following of Christ, is bound to be imperfect.

I would say then that what we can best take from what Matthew offers us is that we must serve Christ as well as we can, as generously as we can follow him in the Spirit, and not worry about our becoming perfect.  That is God's concern and a gift that he gave only to Jesus and Mary.  We need to concentrate on doing what we can, using our time and our gifts as best we might.  Lent is indeed a time of focussing our lives, of purification and re-centering; it is not a time for negativity and despair but is, as the first preface for Lent says, a "joyful season."