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What you describe is the phenomena of the monkey-like mind, which changes more quickly than the body.
From SN 12.61:
“It would be better, bhikkhus, for the uninstructed worldling to take as self this body composed of the four great elements rather than the mind. For what reason? Because this body composed of the four great elements is seen standing for one year, for two years, for three, four, five, or ten years, for twenty, thirty, forty, or fifty years, for a hundred years, or even longer. But that which is called ‘mind’ and ‘mentality’ and ‘consciousness’ arises as one thing and ceases as another by day and by night. Just as a monkey roaming through a forest grabs hold of one branch, lets that go and grabs another, then lets that go and grabs still another, so too that which is called ‘mind’ and ‘mentality’ and ‘consciousness’ arises as one thing and ceases as another by day and by night.
“Therein, bhikkhus, the instructed noble disciple attends closely and carefully to dependent origination itself thus: ‘When this exists, that comes to be; with the arising of this, that arises. When this does not exist, that does not come to be; with the cessation of this, that ceases. That is, with ignorance as condition, volitional formations come to be; with volitional formations as condition, consciousness…. Such is the origin of this whole mass of suffering. But with the remainderless fading away and cessation of ignorance comes cessation of volitional formations; with the cessation of volitional formations, cessation of consciousness…. Such is the cessation of this whole mass of suffering.
OP wrote:
Even thoughts are changing, It feels like source of it remains unchanged. How to understand this clearly? What are the practises that I should do? What meditation practises help to understand this?
Please try the meditation practice taught by Ven. Yuttadhammo in his booklet "How To Meditate" and also taught in this video playlist.
I quote from Chapter One of "How To Meditate":
The basic technique of meditation that we use to facilitate this change is the creation of clear awareness. In meditation, we try to create a clear awareness of every experience as it occurs. Without meditating, we tend to immediately judge and react to our experiences as “good”, “bad”, “me”, “mine”, etc., which in turn gives rise to stress, suffering, and mental sickness. By creating a clear thought about the object, we replace these sort of judgements with a simple recognition of the object as it is.