There are times when I feel that I have got it whereas there are days when I feel that I don’t understand anything. Now I know I am always the unchanging self and it’s my mind that keeps going in and out of Vedanta. How do I establish myself in the true knowledge?
How can I keep on being open to the true reality (Advaita Vedanta)? If I try really hard I can experience/feel/understand it for a short time, but soon after strong wordily desires and ego take over.
Yes, it is not a quick process. It requires gradual development of mind, concentration and experience. In Vedanta, this practice or Sadhana are instucted to be accomplished by four stages which are Sravana, Manana, Nididhyasana and Samadhi.
Sravana means hearing/listening truth from Guru. And by logical reasoning, verifying or reflecting the meaning (of what is listen) in mind is called Manana.
As stated in Panchadasi of Vidyaranya:
इत्थं वाक्यैस्तदर्थानुसन्धानं श्रवणं भवेत् ।
युक्त्या सम्भावितत्वानुसन्धानं मननन्तु तत् ॥ ५३॥1.53 The finding out or discovery of the true significance of the identity of the individual self and the Supreme with the aid of the great sayings (like Tattvamasi) is what is known as sravana. And to arrive at the possibility of its validity through logical reasoning is what is called manana.
Nididhyasana is profound medition on the self and when the self becomes steady and jogtrot/identical with the objective (Brahman), it's called the state of Samadhi as explained in following verse of Panchadashi :
1.54. And, when by sravana and manana the mind develops a firm and undoubted conviction, and dwells constantly on the thus ascertained Self alone, it is called unbroken meditation (nididhyasana).
1-55. When the mind gradually leaves off the ideas of the meditator and the act of meditation and is merged in the sole object of meditation. (viz., the Self), and is steady like the flame of a lamp in a breezeless spot, it is called the super-conscious state (samadhi).
This state of Samadhi is also mentioned in Bhagavad Gita 6.19
So, these are the steps or stages of establishment. And since this is not quick progress or the self is not attained easily, Sravana, Manana, Nididhyasana are needs to be practiced repeatedly and continuously as preached in the following verse of 7th chapter:
7-97. From the great Sayings a direct knowledge of Brahman is obtained, but it is not firmly established all at once. Therefore Sri Shankaracharya emphasises the importance of repeated hearing, reflection and meditation.
One reason is for repetition of mediation and for concentration/control of mind is the property of mind that is unsteady, turbulent, strong and obstinate as inquired by Arjuna in Bhagavad Gita:
6.34 For, O Krsna, the mind is unsteady, turbulent, strong and obstinate. I consider its control to be as greatly difficult as of the wind.
That's why it's also instructed to study with the inner control (Shama and Dama):
7-98. "Until the right understanding of the meaning of the sentence 'I am Brahman' becomes quite firm, one should go on studying the Shruti and thinking deeply over its meaning as well as practising the inner control and other virtues."
In the Bhagavad Gita, Chapter 6, Shloka 35 Lord Krishna says to Arjuna (Translation by Swami Muktananda) :
Asamsayam mahabaho mano durnigraham calam
Abhyasena to kaunteya vairagyena ca grhyate.BG 6.35: Doubtless O mighty-armed, the mind is restless and hard to control but by practice and non-attachment, O son of Kunti, it can be controlled.
The problem which you shared is common amongst all of us, especially with "mumukshu". With focus, we can manage to experience it but a short time, but it's perfectly okay. Knowing self is difficult and at the same time it's very easy.
bandhur ātmātmanas tasya yenātmaivātmanā jitaḥ
anātmanas tu śhatrutve vartetātmaiva śhatru-vatBG 6.6: For those who have conquered the mind, it is their friend. For those who have failed to do so, the mind works like an enemy.
If you are available or aware of things around you then it is very easy, and if your mind is somewhere else and you are not available to present, then it is difficult.
Now, how to be stable in your true self is "Pranayama", if you need to be always alert or focused about your self.
yogī yuñjīta satatam ātmānaṁ rahasi sthitaḥ
ekākī yata-chittātmā nirāśhīr aparigrahaḥBG 6.10: Those who seek the state of Yog should reside in seclusion, constantly engaged in meditation with a controlled mind and body, getting rid of desires and possessions for enjoyment.
If above these looks complex, then just remember few Mahāvākyas always:
Aham Brahmasmi - I am Brahman
Tat Tvam Asi - You are that
Prajnanam Brahma
Ayam Atma Brahma
The technique is "Fake it till you make it". Remembering these will bring your attention on your self. Meditate for few minutes on any of these regularly.
yuñjann evaṁ sadātmānaṁ yogī niyata-mānasaḥ
śhantiṁ nirvāṇa-paramāṁ mat-sansthām adhigachchhatiThus, constantly keeping the mind absorbed in me, the yogi of disciplined mind attains nirvāṇ, and abides in me in supreme peace.
Om Puurnnam-Adah Puurnnam-Idam Puurnnaat-Purnnam-Udacyate
Puurnnasya Puurnnam-Aadaaya Puurnnam-Eva-Avashissyate ||
Om Shaantih Shaantih Shaantih ||Om, That (Outer World) is Purna (Full with Divine Consciousness); This (Inner World) is also Purna (Full with Divine Consciousness); From Purna comes Purna (From the Fullness of Divine Consciousness the World is manifested), Taking Purna from Purna, Purna Indeed Remains (Because Divine Consciousness is Non-Dual and Infinite). Om Peace, Peace, Peace.
During your meditation or even you are dealing with wordily pleasures, avoid the feeling of incompleteness to expedite your progress. You were poorna (Complete), you are poorna and you will be poorna only.
