John Gill Commentary Deuteronomy 31:8

"And Jehovah, he it is that doth go before thee; he will be with thee, he will not fail thee, neither forsake thee: fear not, neither be dismayed." — Deuteronomy 31:8 (ASV)

And the Lord, he [it is] that does go before you
The Word of the Lord, his Shechinah, according to the above Targum, and so in the next clause; the same that brought Israel out of Egypt, had gone before them in the wilderness, and now would go before Joshua and them into the land of Canaan:

he will be with you ;
to guide and direct, to assist and strengthen, to protect and defend, to give success to his arms, and victory over his enemies:

he will not fail you, neither forsake you ;
not fail to give him counsel and direction, to afford him strength, and to fill him with courage, and to deliver his enemies into his hands; nor forsake him till he had finished the work he was to do, had made a complete conquest of the Canaanites, and settled the people of Israel in their land:

fear not, neither be dismayed ;
at the number and strength of the enemy, nor at any difficulties that might lie in the way of finishing so great an undertaking, since the Lord would be with him; see (Romans 8:31) .