Why the log? It feels like homework.
Because memory is a bad witness for stress. Without the log you'll believe everything sets you off equally, when it's usually two or three specific things.
What if the trigger is my job?
Then breathing exercises are a coping tool, not a solution. Use them, and be honest in the debrief about what the job is actually costing. Some problems need a change, not a technique.
Is a cold shower necessary?
No. It's one convenient way to practise staying composed while your body objects. A hard conversation does the same job with more transferable value.
How much can I expect to change in eight weeks?
Recovery speed changes noticeably for most people. Baseline reactivity moves more slowly and less dramatically. Anyone promising you a different temperament in two months is guessing.