The future is frightening because of all the complex possibilities and unknown quantities. Kierkegaard compares this feeling to vertigo. Sartre’s “nausea” is similarly a reaction to the inconceivable complexities of reality. Our certainties dissolve in the imponderable mess of connections surrounding them.
Abstract thought involves ignoring the mess in which “things” are located. In a sense it is a betrayal of that which is being ignored. It is Bad Faith. Being authentic entails personal involvement, which makes one aware of the giddy complexity around and within one.
Existentialism exposes the complexities which the established thought-systems have a vested interest in obscuring. In Sartrean terms, the inauthenticity of Science and Religion lies in their dressing up the slimy mess of reality as discrete, clean entities.
In thinking abstractly, we are trying to avoid the vertigo and nausea of disorientation. We stand aside and try to be objective. But ultimately, truth is subjective. We know we are experiencing it when we feel the fear and discomfort described above.