Your Strength
In 2 Samuel 13-18, David is broken; he's a non-character: he stops being an actor and becomes a prop. 

But it's still David’s story; only now it is told through the story of others. And there is only one character in these chapters who interacts with David in a good way. Take note:

2 Samuel 15:18-21. 
All [David's] servants passed by him...and all the six hundred Gittites who had followed him from Gath, passed on before the king. Then the king said to Ittai the Gittite, "Why do you also go with us? Go back...and may the LORD show steadfast love and faithfulness to you." But Ittai answered the king, "As the LORD lives, and as my lord the king lives, wherever my lord the king shall be, whether for death or for life, there also will your servant be."

From Absolom to the elders of Israel to those who mock David, the only one put forward in the text as a noble character who treats David appropriately is Ittai. The Gittite from the Philistine town of Gath. 

Other noteable residents of Gath include: GOLIATH!

What?! So, after David killed Goliath, Ittai quit as a commander of the Philistines to serve David, along with his troops.

Ittai is a strong person. But when Ittai saw Goliath drop, he knew "there is a God in Israel...and the battle is the LORD's!" (1 Sam 17:46-47) So he left off worshiping Dagon, etc., and joined David. He saw the hand of the Lord on David. So when people are turning on David or deserting him, Ittai stands fast. Ittai understands.

A few chapters later, Ittai and his 600 fight alongside the armies loyal to David and defeat the forces under Absalom. Ittai stands with God's anointed-one.

Here's the point: Ittai is strong. Ittai uses that strength to strengthen David.

Sometimes we feel weak. But in some things we know we are strong. Sometimes we feel stupid. But in some things we know we are smart. Strength and smarts and skill and savvy, for what? For God's King. For Jesus.

Our fighting strength, strategic wisdom, experience, our 600+ warriors--all of it--is for Jesus. Our skills, gifts, our money, time, our creativity, and our energy--all of it--is for Jesus.

2 Samuel 13-18 has one (1) good example, and it's the example of a strong, smart, skilled, savvy, resourced Alpha who says, "Sink or swim, I'm all in."

What do we do with our strength?

There once was a rich, young, smart, strong, educated nobleman.  He had a good heart; he was spiritually sensitive. He met with Jesus and asked Jesus what his thoughts were on how to live a godly life. Jesus said, "Give up your strength." Jesus didn't say, "Give me your weakness," because the young man didn't have much of that. Jesus said, to the strong-man, give me your strength. 

And the strong man wouldn't. (Mark 10:22)