# may fulfill his word

"do everything he promised he would do"

# If your descendants ... you will never cease

Yahweh is talking to David, so the words "you" and "your" refer to David.

# to walk before me faithfully

Yahweh is talking to David, so the word "me" refers to Yahweh.

# with all their heart and with all their soul

"with all their being" or "with all their energy"

# you will never cease to have a man on the throne of Israel

"your descendants will never cease to be kings of Israel" or "one of your descendants will always be the king of Israel"