The second beatitude pivots around another word with negative connotations. "Blessed are they that mourn: for they shall be comforted." Mourning usually carries the idea of survivors grieving after a death. But here mourn has a different meaning.
Broadly speaking, mourning is the emotion of sadness. In this beatitude Jesus puts value on sadness. Sadness, he says, will bring blessings. What type of sadness? In the first beatitude, you were instructed to become humble, small in your own eyes, and start intensely relying on God.
To get to that point, you reviewed your past and found it lacking. Past decisions and experiences have not brought the joy and hope you wanted. Looking back at those mistakes brings unspoken regrets, "If only. . .." These regrets are the sadness, the mourning, of this beatitude.
Your sadness about what has been lost - confidence, peace, joy, and hope - led you to the broken spirit of the first beatitude. Your brokenness gave you a changed attitude about God. Now it turns you to a changed attitude about life. True mourning is regretting past sins, damage done to others, and time wasted ignoring God. The result can be self-pity, or stoic endurance, or resolution to seek change.
Resolving to change your ideas and attitude is repentance. It is both turning away from the past and turning toward a new beginning. When you do this, God promises to bless you. He will actually come to you, comfort you, and let you feel his love.
Forgiveness removes guilt. This is both the blessing and the comfort. When God puts his love into your heart, he lets you know you are forgiven. Your guilt for all your sins and mistakes is instantly removed. You are comforted, you are blessed, and you know God is real.
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