"mounting trepidation" is a correct and usable phrase in written English.
You could use it to describe a rising sense of fear or apprehension. For example: With every passing day, I felt my mounting trepidation about the upcoming exam.
Last week, I read an interview with David Cameron in the Catholic Herald and felt mounting trepidation.
I watch with mounting trepidation as the wobbly pink mounds, resembling gruesome props from some horror movie, are brought to the table.
Take two decadent Yankees, goes the recipe, send them abroad in search of kicks and watch with mounting trepidation as their blindness to cultural nuances, their first-world illusions of invulnerability and their reckless sensuality lure them into dark and fatal corners where their traveler's checks and consulates can't save them.
Ordinary Libyans too have watched with mounting frustration and trepidation as those militias have prevented the country from getting back on an even keel.
With trepidation at first, then with a mounting bounce to their stride, residents who had lived under nearly constant curfew for more than a month stepped out of shuttered homes over battered streets.
One reason for trepidation was the possibility of Mr Esser mounting a "barbed-wire" defence, mobilising politicians and unions against the British company.
With unease mounting here in the capital, the student rallies were watched with trepidation by reform leaders.
Through the evening, as the evidence mounted for a no vote – which, in the confusing world of current Greek politics, was essentially a yes for the stance taken by Tsipras in the talks with EU and international institutions so far – it was possible to sense the trepidation that has gripped the rest of Europe for the past week mounting too.
