There's three kinds of willpower: will, won't, and want. (K. McGonigal, 2012)
It's hard to see which one you need in this situation, but they all have to be trained.
Will is the ability to do something, e.g. getting a shower or exercising daily. You train it by doing uncomfortable things like exercise.
Won't is stopping something, e.g. not yelling at the annoying coworker and not drinking alcohol after a rough day. A simple exercise would be leaving a candy bar at your desk.
Both can be practiced with meditation. Will by initiating the session, won't by focusing.
Also in the book Peak Performance (Stulberg, Magness), it's said that the main difference between the elite and non-elite is that the elites shrug off pain faster. Everyone feels the same level of pain, but the elites don't let it stop them. Meditation is good for that too. You acknowledge that it's pissing you off, but also that the right response is to keep going, instead of reacting immediately to the pain.