It is as you say and there is a certain element of inertia in it; the slowness of the mind and the nature to seize something new to it, the non-distinction between what is true and to be held and what is not true and not to be held, is due to a certain absence of quickness of movement in the being. But each human quality has its advantages and disadvantages.
A quick mind is often unstable—it catches but does not keep; or it catches but only superficially and thinks it has got everything when it has got only a little and not enough. A slow mind that takes slowly but holds on to what it has got, can be slow but sure in its movement. The disadvantage of it is obstinacy, unwillingness to admit what it should receive, unwillingness to let go what is mistaken. Its advantage is steadiness, a firm hold when it gets the right thing. Therefore you should not mind if it takes long to absorb and hold the new consciousness—as a matter of fact, to hold takes long with everybody. Once you have got it well established, your nature is likely to hold it firmly.
As for the lack of discrimination, that is only in the physical and lower vital mind—within you there is something that can discriminate, the psychic. The only thing is to get it out and keep it in front.When you had the psychic state or rather a touch of it from time to time, you saw things very clearly. When the psychic state fixes itself, that discrimination also will become a part of the nature.
Ref: Letters on Yoga – IV