Radak

The Lord is the portion of mine inheritance and my cup: – I have no other portion beside Him as these men have, whose inheritance and portion is silver and gold and the pleasures of the world; but, as for me, the Lord is the portion of mine inheritance and my cup. The portion, the inheritance, the cup, and the lot are identical; it is simply a repetition of the idea with a change of terms to give emphasis, as is the custom of the language; the meaning being, in all my words and in all my affairs He is my inheritance and to Him is my devotion (directed). And the interpretation of Thou maintainest my lot (is): – Thou didst help me when I chose this inheritance for myself, as they say (Babli, Yoma 38b): “He who comes to be cleansed is helped (by Heaven).” And the word, תומיך (maintainest) is written with hireq equivalent to sire; and so “Behold, I add (יוסף) to thy days” (Isaiah 38:5); “Behold, I add (יוסף) to do a marvellous work” with hireq equivalent to sere (ibid. 29:14).