Matthew 5, as you have probably noticed, has many little sections. Almost each paragraphs is a new topic. We can helpfully narrow it down two sections, verses 1-20 and 21-48. The first section does not seem to be connected, but the second is connected by the repeated phrase, “You have heard that it was said.”
The Beatitudes (2-11) do not seem to be an introduction. It is a broadside into our life and thinking. If the beatitudes are an introduction, it’s a mighty introduction, that shouldn’t be downplayed. The first two (verse 3 and 4) are difficult, the rest are thankfully self-explanatory.
This link, to me, is the best and most helpful way to understand verses 3 and 4. It’s worth reading. I think you’ll find it encouraging.
https://www.desiringgod.org/messages/blessed-are-the-poor-in-spirit-who-mourn
The Men's Bible Reading Idea 