てならない

てならない says a feeling is so strong the speaker can't hold it back: "I can't help feeling ~", "terribly ~". It follows the て-form of a verb of feeling or thinking; an い-adjective takes くてならない, and a な-adjective or noun でならない.

It takes only feelings and thoughts that come on their own – worry, sadness, loneliness, a nagging thought – never a deliberate action: ×勉強してならない. The feeling is the speaker's; for someone else's, add ようだ or らしい.

With 思える and 気がする

Spontaneous verbs of thinking – おもえる思える, おもいだされる思い出される, きがする気がする – are where てならない is most at home: a thought keeps coming back whether you want it or not.

てならない, てたまらない and てしょうがない

All three mean "unbearably ~". てならない is the most formal and mostly written. In conversation you hear てしょうがない or てたまらない, and those two also take physical sensations such as heat and hunger, which てならない seldom does.

てならない or てはならない

てはならない, with は, is a different pattern: "must not". Leaving は out turns a prohibition into a feeling.