Writing Consistently When You Have a Day Job

The most useful thing I ever read about writing on the side was that it does not scale with how much you love it — it scales with how much time you protect for it. Enthusiasm is a fuel that runs out around week three of any new project. Time on the calendar does not.
For me the block that works is early morning, before email is open and before anyone else in the house is awake. Not because mornings are magical, but because they are the only slot no one else wants.

The second thing that helps is lowering the daily bar until it is embarrassing. Twenty minutes. Two hundred words. A single paragraph if the day is bad. Consistency beats intensity by a wide margin over any timescale longer than a month, and the goal of a side writing practice is not to sprint — it is to still be writing next year.
I keep a very short list of active projects — usually two — and everything else goes into a "someday" file that I look at once a quarter. Most of what is in that file stays there, and that is fine.

If you are trying to build a writing habit and it keeps collapsing, ask yourself whether you are trying to write and edit in the same session. That is the mistake I made for years. Drafting and revising use very different parts of the brain, and asking a tired weeknight self to do both is a good way to produce nothing.