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-vat
BG 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 aparigrahaḥ
BG 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
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 adhigachchhati
Thus, 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.