1)
Who tells his sin, and to whom?
1.
Rashi: My heart tells me - therefore, "I am worried due to my sin", lest it cause me pain and to limp!
2.
Radak: I say to myself - perhaps I will die from it!
3.
Malbim: David publicizes it to others.
2)
Why is he worried about his sin?
1.
Refer to 38:19:1:1-2.
2.
Sha'arei Teshuvah (1:16): I worry about the punishment for it. For some Aveiros, Teshuvah suspends, and punishments cleanse.