“Kick in the ass” is actually a phrase that I might use (native speaker of Canadian English). Sorry, I haven’t found any references (yet).
but I might use it as follows “my boss gave me a kick in the ass that got me started on the project that I was procrastinating about, and it turned out to be easier than I thought“.
Or “getting out of breath when I ran up the stairs was a kick in the ass that got me serious about exercising”
However, a “kick in the ass” can be both positive and negative, as so many things are in English. Most of the definitions that I have found emphasize the negative aspects, like “my boss threatened to fire me, which was a kick in the ass“. But that in itself might inspire you to start working harder. Tough love.
Also not to be confused with “kick ass“ or “kick butt”.
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Less colloquially, you already mentioned “push”. E.g. “my wife pushed me to start the project of cleaning out the garage”..
More: “trigger”, “prompt”.
“nagged” as you “my wife magged me until I started working on the project”
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IMHO terms like “boost” are not necessarily a thing that start you on a project, although they might help you along the way.
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More figurative:
“Pushed me off the cliff” or “pushed me into the water”
E.g. “my friend pushed me off the cliff into the ocean, where it was sink or swim. Fortunately I swam… and that’s how I got started on my career.”
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Riffing off one of the other answers “I lit a fire under his ass, and he finally started working on the project”.