Rashi citing Mechilta Bo 12: "U'Menuchah Lo Matzasi" - Shechinah ¹ did not dwell on me (Baruch) to prophesize, like [it dwelled on others] "va'Tanach Alehem ha'Ru'ach" (Bamidbar 11:26). Why am I different than Talmidim of the Nevi'im? Yehoshua served Moshe, and Ru'ach ha'Kodesh dwelled on him. Elisha served Eliyahu, and Ru'ach ha'Kodesh dwelled on him!

Radak citing a Midrash: Mach'ov is that Yisrael pain me with disgrace. Yagon is that he himself did not receive Nevu'ah, even though he prepared himself for it as much as he could. He did not have natural preparation for it at the time of his formation, like I explained above (refer to 1:5:1:3*).

Malbim: He prepared himself for Nevu'ah and did not receive it; this pained him. At the time of writing it was a doubled pain. He served the Navi to write his Nevu'ah, and he never had a prophetic arousal like is proper for a spiritual person standing in front of a Navi at the time of Nevu'ah. Even though he was a Kli that helped to put Hashem's word in a Sefer, he should have had Nevu'ah, like we find with Bnei ha'Nevi'im. The episode with Sha'ul proves this (Shmuel I, 10:10. When he came among Nevi'im, also he prophesized.) He saw that even when Yirmeyah prophesizes in front of him, he does not get prophecy - all the more so he will not merit it at another time!

Metzudas David: I am weary from constant groaning and worry over the coming punishments.

Malbim: I made great preparations to find Devar Hashem [but I did not find it].