A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
psa 43:1Excepting the recurrence of the refrain, there is no good reason to suppose this a part of the preceding, though the scope is the same. It has always been placed separate. (Psa 43:1-5)
Judge--or, "vindicate" (Psa 10:18).
plead, &c.-- (Psa 35:1).
ungodly--neither in character or condition objects of God's favor (compare Psa 4:3).
psa 43:2God of my strength--by covenant relation my stronghold (Psa 18:1).
cast me off--in scorn.
because--or, "in," that is, in such circumstances of oppression.
psa 43:3light--as in Psa 27:1.
truth--or, "faithfulness" (Psa 25:5), manifest it by fulfilling promises. Light and truth are personified as messengers who will bring him to the privileged place of worship.
tabernacles--plural, in allusion to the various courts.
psa 43:4the altar--as the chief place of worship. The mention of the harp suggests the prominence of praise in his offering.