פירוש על יהושע 1:8

This Sefer Torah.¹⁴This Sefer Torah implies a specific Book of the Torah. Which one? Thus Rashi comments that it refers to Sefer Devorim that was before Him. The Sefer Devorim was before Him.

This. That was said in the text that preceded (verse 7) that Moses commanded you.

And you shall meditate in it. Every expression of הִגָיוֹן in the Torah refers to the [meditation] of the heart,¹⁵Metsudas Tzion says that הִגָיוֹן sometimes refers to speech as it is written: And my tongue will speak (תֶּהְגֶה) of Your righteousness (Tehillim 35:28). as it is said, ‘And the thoughts of my heart before you;’¹⁶Tehillim 19:15. ‘Your heart will give forth fearful thoughts.’¹⁷Yeshayahu 33:18.

So That You Shall Keep. Because through use from your mouth, lest you forget what.

Because then. When you keep doing, etc., then you will succeed.