what does pip describe as a primary concern, a "high mountain above a range of mountains"? why does this concern cause him so much anxiety?

Pip describes his anxiety and unhappiness over Estella's impending marriage as a "high mountain above a range of mountains"..... a feeling that never left him. This concern causes him so much anxiety because it's the end of the dream..... the end of his desire to make Estella his own wife.

As the time wore on, an impression settled heavily upon me that Estella was married. Fearful of having it confirmed, though it was all but a conviction, I avoided the newspapers, and begged Herbert (to whom I had confided the circumstances of our last interview) never to speak of her to me. Why I hoarded up this last wretched little rag of the robe of hope that was rent and given to the winds, how do I know? Why did you who read this, commit that not dissimilar inconsistency of your own last year, last month, last week?

It was an unhappy life that I lived; and its one dominant anxiety, towering over all its other anxieties, like a high mountain above a range of mountains, never disappeared from my view. Still, no new cause for fear arose. Let me start from my bed as I would, with the terror fresh upon me that he was discovered; let me sit listening, as I would with dread, for Herbert's returning step at night, lest it should be fleeter than ordinary, and winged with evil news,—for all that, and much more to like purpose, the round of things went on.