Understanding uncertainty of thoughts and the mind

It's quite easy to see the changing behaviour of the things we see, hear and feel. 1. Appearence between a child and an old man. 2. Good smell and bad smell 3. Having a wound and not Those things are quite easy to see. But it is hard to see the changing behaviour of mind and thoughts. I'm aware of 1. I may angry now. But I will be grateful in next five minutes. (Changing thoughts). (But I'm also getting repitative thoughts) 2. I have different prespective than in my childhood. 3. Knowledge that I have is changing. 4. If cut down the body into pieces, we can not find any "I" 5. I and I in the previous life shoud be totaly different (Can see this clearly if I was an animal.) 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?