Agitation is a tendency to want (desire) something else than our current experience and/or to not-want (resist) part of or the totality of our current experience.
This agitation consists of thinking and feeling. Thinking something is wrong or missing and feeling it.
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The sense of lack is inherent to the belief and feeling that I refers to a mortal body mind, that I refers to a form that is born in time and space. A separate form arises (as that belief and feeling) and what follows is seeking to become whole in order to appease this sense of separation (the sense of lack). The sense of lack is experienced as unhappiness. Which veils the inherent happiness of our true and universal nature.
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Resistance and the past.
In our current experience, thought may arise. Thought refers to past and future. Images arise as past and future. Good and bad follows. We seek the good (which is seeking something other than our experience) and we resist the bad (which is resisting our current experience).
Within thinking there is the hidden belief that I refers to a person, to a mortal human. 
The inquiry into this belief (and feeling) is essential.