niggling feeling

Kazuo Ishiguroの "The Unconsoled" を読んでいます。
Now, for all his good manners, it was clear he was deeply disappointed with me. I found this thought oddly troubling and, as I continued along the roadside gazing at the sunset, began to feel more and more annoyed that I had not proceeded with greater caution over the matter of the Sattler monument. It was true - as I had pointed out to Pedersen - that my decision had seemed to represent the wisest course open to me at the time. Nevertheless, I could not avoid the niggling feeling that for all the limitations on my time, for all the enormous pressures impinging on me, I should somehow by that point have been better informed.

"niggling feeling" の "niggling" はどのような感覚でしょうか？　辞書を見ます。
・Collins Dictionary: A niggling injury or worry is small but bothers you over a long period of time.: ...a niggling worry that the cheap car is also the one that will cause endless trouble.
・Cambridge English Dictionary: to worry someone slightly, usually for a long time: I just can't remember his name - it's been niggling me for a couple of weeks.
・Wiktionary: A small irritating sensation.: Sellafield hasn’t suffered an accident of equivalent scale since the 1957 fire, but the niggling fear that some radioactivity is leaking out of the facility in some fashion has never entirely vanished

"The Unconsoled" は話が一向に進まないと言うか、次から次に訳の分からない関連のない話が続き、読んでいて非常に疲れる、我慢が必要な話の内容です。