I hear in lectures of GM that he sometimes mentioned
in brief this beautiful stage of self-forgetfulness.
I understand the concept in general, and I would
like to hear something more about it so that I could
grasp a little bit better the idea all together.
As far as I understood it was like this:
When the vision of the devotee becomes so fixed on the lila, all of her attention being given to the seva, then the devotee enters the lila as a participant, not just an observer. The individuality is preserved as the participant in the lila, but the sense of identity outside the lila, outside of seva is forgotten.
In addition to what Atmananda said: Often GM talks about going from self-sacrifice to self-forgetfulness as a way of contrasting other spiritual ideals with that of Vraja-bhakti. If I remember these were terms originally used by Srila Sridhara Maharaja. In jnana the goal is to lose the self, but the motive is in some sense a little self-centered: to end suffering. In Vaikuntha the sevaites see a clear distinction between themselves and Narayana. They identify themselves as servants and him as the master, so they are sacrificing themselves with some conscious awareness of difference. In Vraja, the distinction between Krishna and devotees fades and the devotees don't even think about the distinction, and in effect, I suppose, there isn't one. I can't remember what book of SSM has this but it is expressed very nicely there.
Devyah-Pati das,
You can find more about this in Rupa Goswami's Ujjvala-nilamani (different levels of prema), in Caitanya Caritamrita, in Aesthetic Vedanta and in Jaiva Dharma. I also recommend looking through the 'classroom' section of Tattva-viveka and also the Sanga archives.
What I found from the classroom section of TV, quoted by Gandiva dasi, from Jaiva Dharma originally:
"Vijaya, “How many types of mädhurya-rati are there?”
Gosvämi, “There are three kinds: sädhärani, ordinary, samanjasä, good, proper, balanced; and samarthä, boundless. Kubjä is the example of sädhärani-rati. Kubjä desired her personal enjoyment with Krsna and because this rati is rooted in the desire for selfish enjoyment, it is to be shunned as being of an inferior nature. The rati experienced by the queens of Dvärakä is samanjasä-rati, appropriate attachment, because it is evoked through the codes of marital conduct and tempered by social norms. The gopis of Vraja have samarthä-rati, boundless attachment, which thus transcends all social norms and religiosity. It is not the case that samarthä is asamanjasä, improper behaviour; rather, according to absolute transcendental analysis, it is actually ati-samajnasä, harmonious and most proper in all respects.
“Sädhärani-rati, as displayed by Kubjä, may be compared to a valuable jewel; the samanjasä-rati of the Dvärakä queens is like a Cintämani, touchstone gem; and the samarthä-rati of the gopis is like the most precious Kaustubha gem—there is no more valuable object in all the three worlds.”
So according to this example, self-forgetfulness reaches it's full blossom in the samartha-rati of the gopis that is completely selfless in nature. There are degrees of love of God and different degrees of 'self' present in that love depending on the devotees in question.
The gopis' love for Krishna is so selfless that outwardly it may seem like selfishness but actually their love for Krishna transcends all social norms and conventional dharma - and any sense of self preservation. It is boundless. The material 'self' is such a small place to be in, actually, and the more we are able to let go of that false sense of self, the more spacious and giving we can become. By nature, love is spacious, selfless, giving and boundless. And the false sense of self is the opposite, restricting, selfish, taking, bound by the gunas.
Another lila that comes to mind about this subject is the lila of 'Krishna's headache'. Only the gopis are ready to give the dust of their feet to remedy Krishna's headache, while the rest of the devotees hesitate to give their foot dust to be put in Krishna's head out of fear of offense, going to hell, etc. The gopis don't care about going to hell - they just worry about Krishna and his well being. That is self-forgetfulness.
All glories to their prema!