1.
Abundant ones are saying of my soul
Many of the wise ones of Israel, as Ahithophel and others, were of opinion that this punishment came to him on account of the affair of Bathsheba, as was actually the case; and they were thinking likewise that the kingdom would never return to him; therefore they followed Absalom and rebelled against him, and were saying that for him there was no portion in the world to come.Therefore he says of my soul.
2.
Selah:
The majority of commentators say of the word Selah that it is equivalent to לעולם ("for ever"). And in this sense the word is common in our prayers. And the learned Rabbi Abraham ben Ezra - may the memory of the just be blessed ! - has interpreted (ad loc.) that it is equivalent to אמת (truth). For myself, I say it is not a significant word. And it is to be interpreted as having the meaning "lifting up" (of the musical accompaniment) from (the passage) "cast up, cast up (סלו), signifying that at the point where this word is mentioned and read there was a raising of the sound of the music (מזמור). And the proof is in the fact that it is not found except in this book and in the Prayer of Habakkuk, which was a song (accompanied by music), as is written also there (3:19), "For the chief musician on my stringed instruments."