Hello Lang,
Yes. Nibbana is a difficult to understand for many. The reason for that is that one needs to have knowledge/belief in rebirth and the wider world view of 31 realms in order to understand Nibbana.
Contrary to what most people believe, repeated rebirth in this world of 31 realms has much more suffering than any short periods of happiness, in the long run.
For most of us in the human realm happiness seems to be more frequent than any suffering. There is even more happiness in deva and brahma realms.
However, living being spend much more time in the four lowest realms compared to time they gte to spend in human and higher realms. This may be hard to believe, but the Buddha said that it is indeed the case; see, “How the Buddha Described the Chance of Rebirth in the Human Realm“.
– That is because most beings do immoral things to enjoy life, and anyone who has not attained a magga phala CAN BE tempted to immoral things if the temptation is high enough.
Now, Nibbana and “this world of 31 realms” are totally detached. The Buddha said that Nibbana exists, but it cannot be described by the terminology of “this world”. Furthermore, and most importantly, there is no a trace of suffering in Nibbana.
When one attains Parinibbana, one will be totally removed from this suffering-filled world and one will be in Nibbana. More details at, “Nibbana“.
This is a bit easier to understand in terms of the four ultimate entities: rupa, citta, cetasika, and Nibbana.
– The first three belong to “this world” and Nibbana is totally detached from “this world”. In other words, Nibbana and “this world’ are mutually exclusive. One can be either in “this world” or in Nibbana.
– So, it is true that citta, cetasika, and rupa do not exist in Nibbana.
But other than the fact that there is no suffering in Nibbana, we do not know what would be the “experience of Nibbana”. Only those who can get to Nirodha Samapatti can “experience” Nibbana while in this world (only up to 7 days).
– Even though beings in the asanna realm do not experience citta vithi as you mentioned, they “come back to this world” when their lifetime expires. Thus even though suffering stops for the duration of that lifetime, their suffering does not end.
On the other hand, when one merges with Nibbana (at Parinibbana), there is no coming to “this world” that is filled with suffering. Suffering is stopped permanently.