Agree with the above - it's the service ticket from which you work. Divide that big job into the little jobs that go into it (and for which you eventually bill the custormer).

Then, a momentary break between all the little jobs. Seems silly, but on days I was assigned bike building/assembly, I stuck a packet of peanut M&Ms in the breast pocket of my apron or service clothes and allowed myself one after every bike. Kept me focused. You'll have to find out what works for you. Talk to the boss.