Unflinching devotion and yoga of exclusive loyalty to Me (the Teacher who exposes the supreme Truth); the habit of seeking seclusion and finding joy in it; lack of interest in and disinclination to be with crowds and the din and bustle of the world.
Krishna is the spiritual Teacher. Arjuna his disciple. Arjuna had made this clear right in the beginning (2.7). The devotion Krishna refers to is of the seeker to his Teacher. It should be unswerving. Krishna uses a strong word: non-desultory devotion.
It means, any kind of dilution or corruption in this will be gravely disharmonious. In spiritual seeking, everything depends upon one’s Guru. He alone reveals the Truth, which, though reigning within the body, is imperceptible to the senses. Faithful listening and following the Teacher’s words has ineffable effect.
Love to be in secluded places. Living in Ashrams for a spell periodically is very helpful. Avoid crowds and multiple interests and indulgences. Filter your habits and choices. Narrow down to spiritual seeking and inner application. Spiritual progress will then be faster.