Master said that "Ego is the Soul identified with the body" but what about the mind? Did Master include the mind as part of the body in this statement? As in "Ego = the Soul identified with the body + mind" (and its contents like thoughts and emotions especially negative ones)? Isn't (the content of) our mind as illusory as our body?
I am asking because it is quite "easy" for me to realize that I am not my body but it is so difficult to realize that I am not my negative thoughts and emotions. Why is that?
Your intuition is correct: when Master said the “Ego is the Soul identified with the body” he includes the mind with the body. In fact, as you may know from your studies of the yogic teachings, we have three bodies! The soul arises from the thought (or ideational or causal) body while the ego arises in the astral body.
And yes, our identification with our thoughts and emotions, memories, fears, and subconscious impressions and tendencies: all of these are a part of the “body.”
Because the mind is so much more subtle than the physical body, and because it is closer to the soul (being astral) than the physical body, it is much more difficult to disassociate our “Self” from the mind.
There are two basic approaches to breaking the hypnosis of self-identification with thoughts and feelings: 1) Mindfulness; watching the thoughts (without reacting to them), and 2) Focusing our attention upward in the positive direction of the soul. This includes devotion to God, Guru, or any impersonal manifestation of the soul such as peace, love, calmness, communion with AUM, and merging with the Inner Light at the point between the eyebrows. This second approach is recommended as best both by Patanjali and by Yogananda and many other great preceptors. Krishna in the Bhagavad Gita says that the I-Thou relationship is much easier for embodied beings.
The power to know the Soul as our true Self comes with the combination of self-effort and divine grace, including the grace of the Guru.
I hope this is helpful.