Why does it say "mi"Ketzei ha'Aretz"?
Rashi: Even though I am far from my men that I send to war against my enemies
What is the meaning of "Elecha Ekra ba'Atof Libi"?
Rashi: My heart is [full of concern for] over them. I call to You to "put me on a rock higher [and stronger] than me."
Radak: Much Tefilah at a time of affliction is called Atifah - "Tefilah l'Ani Chi Ya'atof v'Lifnei Hashem Yishpoch Sicho" (102:1).
Malbim: My heart was wrapped due to the many afflictions.
Why did he request a rock taller than himself?
Radak: I cannot ascend it; You will put me there! This refers to Eretz Yisrael, whether the verse discusses David or Galus.
Malbim: He did not; the verse says that he called to Hashem when he needed to ascend on the high rocks of Ye'elim to hide.