Our essence

rādhā kṛṣṇera līlā ei ati gūḍhatara;
dāsya vātsalyādi bhāvera nā hoy gocara
sabe eka sakhīgaṇera ihā adhikāra;
sakhī hoite hoy ei līlāra vistāra
sakhī vinu ei līlāra puṣṭi nāhi hoy;
sakhī līlā vistāriyā sakhī āsvādoy
sakhī vinu ei līlāra anyera nāhi gati;
sakhī bhāve tāre jei kore anugati
rādhā kṛṣṇa kuñja sevā sādhya sei pāya;
sei sādhya pāite āra nāhiko upāya
sei gopī bhāvāmṛta jāra lobha hoy;
veda dharma loka tyāji kṛṣṇa bhajaya
rāgānugā mārge tāra bhaje jei jana;
sei jana pāya vraje vrajendra nandana
vraja lokera kona bhāva loiyā jei bhaje;
bhāva jogya deha pāiyā kṛṣṇe pāya vraje
ataeva gopī-bhāva kori aṅgīkāra;
rātri dine cinte rādhā kṛṣṇera vihāra
siddha dehe cinti kore tāhāi sevana;
sakhī bhāve pāya rādhā-kṛṣṇera caraṇa
gopī anugata vine aiśvarya jñāne;
bhajileho nāhi pāya vrajendra-nandane
(Caitanya Caritamrita Madhya 8)

Translation –
“Rādhā-Kṛṣṇa’s confidential līlās are unknown to the bhaktas situated in dāsya, sakhya or vātsalya rasa, what to speak of others; only the sakhīs are eligible to know them. As these pastimes are solely nourished, expanded and relished by the sakhīs, apart from following in their footsteps through gopī-bhāva, eligibility for Rādhā-Kṛṣṇa’s nikuñja sevā is inaccessible. Those anxious to achieve Kṛṣṇa’s amṛtamaya sevā as a Vraja-gopī should discard unrelated sections of the Vedas, and the opinions of disinterested persons. Then by worship on the rāgānugā-path, Vṛndāvana’s Kṛṣṇa will be attained, i.e., by following the moods of a particular Vṛndāvana resident, a similar siddha deha will be awarded. Therefore accept gopī-bhāva, and contemplate Rādhā-Kṛṣṇa’s pastimes throughout the day and the night. Conceive of your siddha deha and perform mānasī sevā within that mentally conceived form. Thus you will attain Rādhā-Kṛṣṇa’s caraṇa sevā as a Vraja-gopī. If one neglects to follow in the gopīs’ footsteps, but remains attached to the path of aiśvarya (reverential worship, vidhi bhakti) Vṛndāvana’s Kṛṣṇa is unattainable.”

The manjari bhava sadhakas know very well that the secret here is the following:
The gopi/sakhi-bhava Sriman Mahaprabhu speaks of herein is nothing less than glorious manjari bhava.

anarpita-carim cirat karunayavatirnah kalau
samarpayitum unnatojjvala-rasam sva-bhakti-sriyam
harih purata-sundara-dyuti-kadamba-sandipitah
sada hrdaya-kandare sphuratu vah saci-nandanah

“May the Supreme Lord who is known as the son of Srimati Saci-devi be transcendentally situated in the innermost chambers of your heart. Resplendent with the radiance of molten gold, He has appeared in the Age of Kali by His causeless mercy to bestow what no incarnation has ever offered before: the most sublime and radiant mellow of devotional service, the mellow of conjugal love.”
(Caitanya-caritamrta Adi-lila 1.4)

Sakhi-bhava was already in existence BEFORE Sriman Mahaprabhu appeared. So what exactly was never given before? Manjari bhava.

My most beloved Gurudeva writes:

The mahājanas say –

sādhana ekhāne, siddhi-o ekhāne,
bhāvera gocara se,
ekhāne tā yadi, dekhite nā pāo,
marile dekhibe ke

“sādhana (practice) is here (in this world) and siddhi (perfection) is here, and it is perceivable at the stage of bhāva. If you don’t see Kṛṣṇa here (due to a lack of bhāva), then how will you see Him when you die?”

The mind of a devotee who is fixed in smaraṇa goes to the kingdom of līlās and relishes the pastimes so vividly there as if they are directly perceived.

siddha deha cinti kore tāhāi sevana;
sakhībhāve pāy rādhā-kṛṣṇera caraṇa
(C.C.) 

Think of your devotional service in your siddha-deha (spiritual body), thus you will attain Rādhā and Kṛṣṇa’s lotus feet in the mood of the sakhīs.

The mahājanas say that each and every soul is eligible for the service of Śrī Rādhā, and by the power of associating with these mahājanas pure God-consciousness spontaneously manifests itself in the pure spirit soul. sei gopī bhāvāmṛta yāra lobha hoy (C.C.) –  “Anyone who becomes greedy for that nectarean mood of the gopīs…”

These people are the fortunate souls who have obtained the grace of like-minded great rasika devotees.
The fullest manifestation of devotion is gopī-bhāva, for the gopīs are exclusively dedicated to the pleasure of Śrī Kṛṣṇa. This gopī-bhāva is the treasure of the practice of the pure swan-like devotees, and the culmination of this sakhī-bhāva is mañjarī-bhāva.
By following in the footsteps of the eternally perfect devotees, that belong to the Lord’s internal potency, one can attain the confidential nikuñja-sevā of Śrī Rādhā and Mādhava.