1)
When did David say this?
1.
Radak: When he heard that they ambush him in all places.
2)
Why did he say "Keli Atah"?
1.
Radak: You are my strength. I have no strength to escape them if You will not help me.
2.
Malbim: This is one of two reasons why I trust in You. (a) You always save me. Also refer to 140:7:3:1.
3)
Why did he now ask Hashem to hear his supplication?
1.
Malbim: I trust in Him because He answers all who call to Him, even if He did not always save them.