1)
Why does it say "Chashti"?
1.
Radak: I was quick and zealous in Mitzvos; I was not lazy in them.
2.
Malbim: Laziness and love of rest did not block me. I did not delay to guard Your Mitzvos. I did a Mitzvah immediately when it came to my hand, even if there was a great exertion - do not Machmitz (ferment) Mitzvos (Mechilta 9).
2)
It already said "Chashti". Why does it add " v'Lo Hismahmahti"?
1.
Orchos Yosher p.32: I went quickly with Zerizus, and not because I delayed beforehand.
