1)
Why did he say "Natisi Libi"?
1.
Radak: If my heart leaned to ways of the world, I tilted it to do Your Chukim. I.e. my desire and thoughts are to do them.
2.
Malbim: Also the Chukim, which have no reason and the heart is far from accepting them, I leaned my heart until it is prepared to do them with desire.
2)
What is the meaning of "l'Olam Ekev"?
1.
Rashi: [My] heel is always on the circuits and paths of Your Chukim. Malbim - this is due to great habituation in them.
2.
Radak: I will always do [Your Chukim], until the end of my days, like I explained "v'Etzrenah Ekev" (refer to 119:33:2:4).
