Oh! You're about 100 or 200 years too late. The 20th century was only the end result of several generations' worth of the erosion of Enlightenment ideals.
Romanticism is a word you'll find helpful. The first inkling of disenchantment with the Enlightenment comes with German proto-romantic writers, such as Goethe. Sorrows of Young Werther had its entire plot set up by a man discarding reason and logic in favour of emotion and feeling. Even in the 1780s, people were tired of the exhalation of reason. By that point it had been in vogue in Europe for about 150 years- Hobbes' career was forged in the 1620s after all.
The Marquis de Sade built his entire ignoble career off of fulfilling the passions of the body and his contempt for those who used reason as an absolute. He goes on and on about how nothing people use "reason" to support actually makes sense unless you've bought into the myth.
Byron and Shelley built up off this disillusionment and made their entire careers on romantic ideals. Going off and dying in a random war of independence because you love Greece so hard you want to LARP as a Democracy-loving freedom fighter?
Byron signed right off on that.
Note Frankenstein, written by Mary Wollstonecraft Shelley. Her mother was a prominent supporter of reason, yet she herself posits reason as the source of evil. The use of the mind to create life should have been a towering achievement of rational thought, but in her book it does nothing but create suffering.
All around Europe, we see people saying "to hell with reason!" and talking about how they feel, man.
So we get the stoner-like rhetoric of the Victorian era, where rational Enlightenment theories on what a moral life should consist of gave way to people proclaiming that you should live your life however you wish- provided it is moral.
Once again, literature of the day speaks of people forgoing finding true happiness doing what they love, or what their heart compels them to do, rather than by doing what is just or even good. Werther kills himself in the end of Goethe's book. Is it rational? Not in the slightest. But is it true to his heart? Absolutely- and that is what matters, according to the Romanticists.
In the realm of aesthetics, thought moved away from whether something was rational to how it made you feel. Paintings like this:
The perspective, proportion, framing, skill in execution… and the pure grace of the city!… how rational.
Gave way to this:
Where a nobleman of the time might think woah, mate, the desolation and the columns… that's so deep… I'm gonna go write a poem about that.
Architecture moved away from this:
I do say, the proportions of the polychrome marble, pilasters and ceiling are so proportional!
Oh God I feel so Roman right now. Let's bust out the togas.
As to how this ties into the 20th century, because moral worth was suddenly ascribed to doing what you felt was true to yourself, people had to wonder why they felt a duty to their nation. And it wasn't because the English state is the most efficiently governed and rationally based, but because it's our destiny man!I feel it!
And thus we get nationalism, where not only do you accept your country, you love it- and if you don't love it, then you're a traitor to the English people. Not only do you live in England- you are England. Someone from the time might say the following:
All throughout history, examples of English greatness abound, and it is so… romantic?… to think about that. English poetry is the best, English architecture is the best, English culture is the best… and thus, in a way, England is actually superior to every other country on Earth. I know it because I feel it. So by not loving your country, you fail to live up to English greatness.
And then the 19th century consists of each European country trying to prove to the others that they are the greatest. The fierce military, scientific and industrial rivalries are all products of nationalist rhetoric.
Then in the 20th century, we just get even more nationalism. Not only are Germans the greatest people in the world, blessed with superior genes, but they are so superior that the inferior peoples are holding us back and must be exterminated in order for the German nation to reach its full potential.
Thus you get Hitler. So by the 20th century, Enlightenment ideals were already long dead, having died quickly and violently before the Victoria era.