The verse should be re-arranged. It means "and I was your father's servant from yore."

Radak: The prefix Vov in va'Ani is superfluous. I was your father's servant from yore." This is like "va'Yisa Avraham Es Einav" (Bereishis 22:4) and many other verses, so people speak in Leshon Arev.

Malbim: I did not change my mind. I planned and still plan to serve both you and your father, and I counseled him to cede his kingship to you.

Malbim: If I cannot persuade David to cede his kingship to you, I will serve you alone, for all Yisrael chose you.

Malbim (34): Meforshim argue about this. Mahari holds that he did not. David was upset when Avshalom was killed, for he knew that Avshalom would not have killed David. He only wanted David to make him king in David's lifetime. This is why he did not accept Achitofel's counsel; Achitofel wanted to kill David. Really, Avshalom wanted to become king however he can - if through David's consent, or by force. Had David consented, surely he would not have rebelled, and he would have honored him like a father and king, like Shlomo honored David even after Shlomo sat on the throne. If Avshalom would force David to make him king, he would prefer not to kill him - unless he feared that David will rebel when he gets the chance. This depended on the counsel of his advisors. David knew that Achitofel will counsel him in the latter way due to hatred for the episode of his granddaughter Bas Sheva. David was wise to send Chushai ha'Archi to be an advisor to Avshalom, not amidst hatred of David, rather, of love for both David and Avshalom. He supports Avshalom for he sees that he is justified. For the good of Yisrael, David should cede his kingship to Avshalom. ¹ He no longer has strength to judge the nation. None of his sons is as proper to succeed him as Avshalom, especially since the nation wants Avshalom!