When you will, make a resolution, set your jaw, you are...

When you will, make a resolution, set your jaw, you are expressing an imaginative fear that you won't do the thing. If you knew you would do the thing, you would smile happily and set about it. And this fear (since the imagination is always creative) comes about presently and you slide down into the complete slump of several weeks or years - the very thing you dreaded and set your jaw against.