# Daughter of my people

"My people, you who are like a daughter to me" or "My dear people". Jeremiah shows Yahweh's love for his people by speaking to them as a daughter. See a similar phrase in Jeremiah 4:11.

# gird yourself with sackcloth and roll in ashes

"Show how sad you are by wearing sackcloth and rolling in ashes"

# gird yourself with sackcloth

"put on the clothes that show you are mourning" or "prepare to mourn". The command "gird yourselves," which usually means "prepare for work" or "prepare for battle" is ironic: they are to prepare to mourn, not to work or fight. See Jeremiah 4:8. 

# mourn with painful sobs as for an only son

"mourn with painful sobs as you would if your only son died". Jeremiah refers to the sadness that a mother feels if her only son dies.

# for the destroyer will suddenly come upon us

"because the enemy army will suddenly come to attack us"