主句：We should live each day.
with gentleness,vigor and a keenness of appreciation 充满精力、热情等等
上面连起来就是说我们应该充满热情,精力等等来度过每一天
which are often lost /when time stretches before us/ (in the constant panorama of more days and months and years to come).which指的是上面所说的精力热情等.
意思是 当时间以更多天、更多月、甚至更多年不变的形式呈现在我们面前时,我们经常会失去热情,精力等.in.to come 是指以某某的景象到来