Mundaka-3-1-2-samāne vṛkṣe

Sri Shankara’s Commentary (Bhashya) translated by S. Sitarama Sastri

In this state of things, the Jiva, i.e., the enjoyer occupying the body as above described, under the heavy load of ignorance, desire and thirst for the fruits of Karma, etc., sinks down like a bottle-gourd in the waters of the sea, is convinced beyond doubt that the body is the atman and thinking that he is the son of this man or the great-grandson of that, lean or stout, with or without good qualities, is enjoying or suffering, and that there is none but him, is born, dies, is united with and parted from relations and kinsmen; therefore, he grieves from helplessness thus: “I am good for nothing; I have lost my son; my wife is dead; what avails my life” and so forth and is subject to anxiety from ignorance owing to numerous kinds of troubles; but when thus constantly degenerating in births, of pretas, beasts, men and the like, he happens, owing to the result of pure deeds stored up in many (previous) births to be instructed in the path of Yoga by some preceptor surpassingly compassionate and being qualified by abstinence from giving pain, truth speaking, continence, complete renunciation and control over the internal and external senses and with his mind concentrated, finds by dint of meditation, the other who is approached by different paths of Yoga and by the followers of Karma distinct from him, conditioned in the body, not subject to the bondage of Samsara, unaffected by hunger, thirst, grief, ignorance, decay and death and lord over all the universe and thinks thus: “I am the atman, alike in all, seated in every living thing and not the other, the illusory atman, enclosed under conditions created by ignorance and this glory—this universe is mine, the lord of all,” then he becomes absolved from grief, i.e., is released entirely from the ocean of grief, i.e., his object is accomplished.

Links
Various resources on the Mundakopanishad
Upanishad Series – Mundakopanishad Commentary by Swami Chinmayananda