Because thou hast been my help - Because thou hast interposed to defend me in danger. The idea is, that he had experienced the divine interposition in times of danger, and that this was a reason why he should still confide in God. The argument is, that God’s mercy and favor in the past is a reason why we should confide in him in time to come.
Therefore in the shadow of thy wings will I rejoice - Under the shadow or protection of thy wings will I feel safe. See the notes at Psalms 17:8. Compare Psalms 36:7; Psalms 57:1; Psalms 61:4.