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 131:1This Psalm, while expressive of David's pious feelings on assuming the royal office, teaches the humble, submissive temper of a true child of God. (Psa 131:1-3)
eyes lofty--a sign of pride (Psa 18:27).
exercise myself--literally, "walk in," or "meddle with."
psa 131:2Surely, &c.--The form is that of an oath or strongest assertion. Submission is denoted by the figure of a weaned child. As the child weaned by his mother from the breast, so I still the motions of pride in me (Mat 18:3-4; Isa 11:8; Isa 28:9). Hebrew children were often not weaned till three years old.
soul--may be taken for desire, which gives a more definite sense, though one included in the idea conveyed by the usual meaning, myself.