The layering system has one job: let you dump heat while walking and trap it the moment you stop. Base layer moves moisture, mid layer traps warm air, shell blocks wind and snow.
Mistake one is cotton. It absorbs sweat, holds it against your skin and turns into a cooling jacket the instant you stop at a pass. Merino or synthetic only, with no exceptions.
Mistake two is over-dressing on the climb. Start each morning slightly cold. If you are comfortable in the first ten minutes you are wearing too much, and you will be soaked by the first tea house.
Above 5,000 m add a belay-style down jacket that you only wear at rest stops and in camp — big, warm and pulled on over everything else the moment you stop moving.