Haridas, I suspected you have taken those statements from the Upanishads out of context.
Katha Upanishad:
1.2.8 The knowing (Self) is not born, it dies not; it sprang from nothing, nothing sprang from it. The Ancient is unborn, eternal, everlasting; he is not killed, though the body is killed.
1.2.12. The wise who, by means of meditation on his Self, recognises the Ancient, who is difficult to be seen, who has entered into the dark, who is hidden in the cave, who dwells in the abyss, as God, he indeed leaves joy and sorrow far behind.
1.2.17 The Person not larger than a thumb, the inner Self, is always settled in the heart of men. Let a man draw that Self forth from his body with steadiness, as one draws the pith from a reed . Let him know that Self as the Bright, as the Immortal; yes, as the Bright, as the Immortal.
1.3.12 That Self is hidden in all beings and does not shine forth, but it is seen by subtle seers through their sharp and subtle intellect.
2.4.4 The wise, when he knows that that by which he perceives all objects in sleep or in waking is the great omnipresent Self, grieves no more.
2.4.5 He who knows this living soul which eats honey (perceives objects) as being the Self, always near, the Lord of the past and the future, henceforward fears no more. This is that.'
2.5.8 He, the highest Person, who is awake in us while we are asleep, shaping one lovely sight after another, that indeed is the Bright, that is Brahman, that alone is called the Immortal. All worlds are contained in it, and no one goes beyond. This is that.
2.5.9 As the one fire, after it has entered the world, though one, becomes different according to whatever it burns, thus the one Self within all things becomes different, according to whatever it enters, and exists also without.'
2.5.10. As the one air, after it has entered the world, though one, becomes different according to whatever it enters, thus the one Self within all things becomes different, according to whatever it enters, and exists also without.
2.5.11 As the sun, the eye of the whole world, is not contaminated by the external impurities seen by the eyes, thus the one Self within all things is never contaminated by the misery of the world, being himself without.'
2.5.12 There is one ruler, the Self within all things, who makes the one form manifold. The wise who perceive him within their Self, to them belongs eternal happiness, not to others
2.5.13. There is one eternal thinker, thinking non-eternal thoughts, who, though one, fulfils the desires of many. The wise who perceive him within their Self, to them belongs eternal peace, not to others .'
There is Advaita written all over this Upanishad:
1) It talks of the self which is hidden in all beings
2) It talks of only one self, which appears to be different because it enters into different things(in Advaita these are called upadhis)
3) It says the self can never be tainted or contaminated, it remains only a pure witness
4) It talks about realizing that self within and attaining supreme happiness, fearlessness
5) It teaches meditation to know the self, by steadying ones mind
6) It equates the self directly to Brahman
It does not say anything about worshiping some separate god. There is no Bheda here.