If you're interested in exploring what you are, here's an exercise I've found very helpful. I call it "Before, During and After." If you've been doing self-inquiry for a while, this will not be new to you; however, you might find it useful as a way to recognize clarity in those instances where you're feeling trapped as the personal, separate entity and identifying with thoughts, feelings and events.
Whatever it is that you are, I think it's accurate to say that it is always present. In other words, what you are doesn't come and go. There is never a point where you can say "I don't exist" or "I'm not here." Something about you is always here, 24/7.
Let's put aside the issue of deep, dreamless sleep for now. You may feel that you can't confirm your existence during deep sleep. But I don't know anyone who claims that their existence stops during deep sleep and that they spring back to life each morning. There's a sense, at least, that what you are is Ever-Present.
By the way, if you think you don't exist during deep sleep, there might be someone sleeping in the room with you, hearing your snoring, that will assure you that you haven't ceased to exist!
In contrast to your Ever-Present nature, all thoughts, feelings, emotions and sensations come and go. They have a beginning and an end. And whatever it is that YOU are... witnesses the coming and going of all phenomena.
Let's take sadness for example.
BEFORE sadness is felt, YOU are there. You know when it has arisen.
DURING the feeling of sadness, YOU are there.
AFTER sadness passes, YOU are there.
Of course the same is true for bliss. Before bliss arises, YOU are there. During bliss, YOU are there. When bliss passes, YOU are there.
What are YOU?
You are that which witnessess all thoughts, emotions, and sensations. YOU are the constant. The phenomena are temporary.
You are Awareness, Consciousness, or whatever name you wish to give to it. You are not a thing. All things come and go in YOU.
Some might think that they are the personality or the characteristics they identify with, such as I'm John, I'm Chinese, I'm a plumber, or I follow such and such religion. However, those things can change and yet YOU don't. For instance, if you changed your name, YOU would still be YOU. The essence of what you are wouldn't change.
If you had amnesia and forgot the country of your birth, would the essence of YOU change? No. If you change careers or religions, get higher or lower self-esteem, or change any aspect of your personality, the essence of YOU would be the same.
YOU are not your personality or any description you might give of yourself. YOU witness all aspects of the personality and descriptions. They can come and go, but YOU always remain.
What can YOU be, other than the Presence/Awareness in which everything comes and goes?
Even the thought of being an individual person comes and goes. Every night when you go to sleep, you give up this thought of being a person. When you wake up, the thought of being a person with things to do that day appears once again.
BEFORE the thought of person arises, YOU are there to witness its arrival.
DURING the thought of being a person, YOU are there.
AFTER the thought of being a person passes, YOU are there.
The thought of being a person comes and goes many times throughout your day. You can be daydreaming or watching a sunset without holding onto the idea that you are a person. Observation and knowing takes place but it is more of an impersonal observation and knowing.
You can go to a hypnotist, who can get you to believe that you are a giraffe, or a tree. Does that change the essence of what YOU are? You don't become a giraffe or a tree by believing that thought. When the belief of giraffe or tree goes away and you re-accept the idea of being a person, has anything about your true essence changed? No, it has been untouched by any beliefs that arose and subsided.
What YOU are never changed.
When you looked in a mirror at age 10, some entity recognized you and knew you existed. When you look in the mirror now, that same entity recognizes you and knows you exist. Every cell in your brain and in your entire body has changed many, many times from age 10 to today. Yet the entity that knew you at age 10 is unchanged today. It has NOT been coming and going.
We tend to get trapped into putting our focus on thoughts, emotions and sensations that are raging at the moment. The mind is enthralled with things and appearances. In so doing, we identify with these phenomena and forget the peaceful background in which all phenomena appear.
Bringing attention back to our true nature (Awareness) brings peace, compassion and insight. Awareness is not personal and is not troubled by any phenomena that appear.
Apply the Before, During, and After exercise to any thought, feeling, emotion -- whether positive, neutral or negative. Realize that what YOU are never changes. YOU are the background, the screen, in which all phenomena come and go.
YOU are the changeless in which all change appears.