What does Pip describe as his primary concern, a "high mountain above a range of mountains"? Why does this concern cause him such anxiety?

This surrounded Estella's impending wedding,

"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."