Great question! (And that’s awesome you’re a copywriter. I’ve studied copy as well — both when doing voiceovers and when writing for myself and other people — and it totally tightens up other writing.)
There’s a quote that I heard somewhere which really made sense to me and it goes something like this: I write to figure out how I feel. Some writers are obsessed with outlining before they start. I sometimes use that, but it’s usually after I’ve engaged with the idea by writing some passages. I’m not sure what I’m really thinking until I’ve written.
And I get the impulse to write when something has pissed me off in ways large and small. I have the thought and then grab whatever paper and pen are near and begin.
If I may suggest a book on writing? How to Write Funny has been so instructive. It gives you strategies to gets laughs. I write a lot of satire in my sketch comedy characters voices (both in video and in print) and even though I have a lot of experience with comedy and good writing, this book has been a recent guide to bump up the laughs even more. Sometimes having concrete structures around writing and help generate ideas.