Jamieson, Fausset and Brown on Psalms 131:2

2. Surely, &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 (Mt 18:3, 4; Is 11:8; 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.