1)
Why did he say "v'Eshmerah Sorasecha Tamid"?
1.
Radak: When Divrei Torah will be regular in my mouth, I will constantly remember them and guard them.
2.
Malbim: This will atone for sin - "b'Chesed v'Emes Yechupar Avon" (Mishlei 16:6). Via this, kingship will remain with me "l'Olam va'Ed."
2)
What is "l'Olam va'Ed"?
1.
Radak #1: It is all the days of my life.
2.
Radak #2: It is in the world to come. I will always guard Torah in this world, and it will be guarded for me forever, i.e. in the world to come.
3.
Radak #3: Your Torah, which exists forever, I will guard it constantly.
