Then, if he feared that she was willing to be his, it must have been because he thought she loved him?
Not a bit of it.

As already stated, he thought he had abundant reason to think otherwise.
It was remorse that he feared might bring her to his feet, the discovery that while she had been gibing at him he had been a heroic figure, suffering in silence, eating his heart for love of her.
Undoubtedly that was how Grizel must see things now; he must seem to her to be an angel rather than a mere man; and in sheer remorse she might cry, "I am yours!" Vain though Tommy was, the picture gave him not a moment's pleasure.

Alarm was what he felt.
Of course he was exaggerating Grizel's feelings.

She had too much self-respect and too little sentiment to be willing to marry any man because she had unintentionally wronged him.

But this was how Tommy would have acted had he happened to be a lady.