How did his parents abandon him?
Rashi (from Vayikra Rabah 14:5): At the time of relations, they intended for their pleasure. Once they finished, they turned their faces away from each other 1 .
Radak: After they raised me (they ceased to support me).
Malbim: David depicts himself as an orphan. His parents abandoned him, and Hashem had mercy on him, and gathered him into His house to raise him.
Chidushei ha'Lev (Vayikra 12:2): Why did David suspect his parents of this? Yishai was one of the four people who never sinned (Shabbos 55b). Surely David assumed that his parents fulfilled "b'Chol Derachecha Da'ehu" (Mishlei 3:6) - one must intent in all his actions for Avodas Hashem, and not for his own Hana'ah (OC 231:1). However, even the greatest Chasid, since he is human, cannot purify his intent without a drop of intent for his pleasure - and this is like sin.
What is the meaning of "Hashem Ya'asfeni"?
Rashi: He guards [the drop of semen] and forms the fetus.
Radak: You prepared my food and needs.
Malbim: Refer to 27:10:1:3.
