At any given moment we can choose to focus on one of two things: what is or what is not.
What is not includes the past, the future, how we want things to be, how we don't want things to be, etc.
When you choose to focus on what is not you engage in an internal struggle against what is -- the present moment.
The present moment is the only moment that exists. The past and the future exist only as concepts in your mind.
According to spiritual teacher Nirmala (www.endless-satsang.com) when people notice their tendency to struggle against what is they often try to change this habit and thus begin struggling with their struggle to accept what is. They judge themselves and think that it is bad for them to have trouble accepting the present moment.
Nirmala suggests that it is enough to simply notice the struggle, for in doing that you are allowing yourself to experience what is happening in the present moment. By simply noticing yourself struggle against what is you are, in fact, accepting what is.
The ego is what causes us to struggle against the present moment. We transcend the ego by recognizing when it is at work. Thus, by recognizing that we are having trouble accepting the present moment (and that the ego is responsible for this struggle) we are learning to transcend the ego and to connect with our true nature.