Rashi: My heart is a Shali'ach for You, and asks for You, that all of Yisrael should seek Your face. I heed [my heart]; I seek Your face! Radak - this is like "Af Leilos Yiseruni Chilyosai" (16:7). My heart chastises me that they should seek You. It counseled me so!

Malbim: Even though my mouth requests other matters, e.g. salvation from the enemy, my heart already said my primary request - that my face seek Your face. This is Devekus (clinging to Hashem)!