quitter; someone who can't stick to anything
Describes a person who gives up easily or never finishes what they start. Literally 'three-day monk', from the idea of someone who enters a monastery but leaves after only three days.
He never sticks with anything; he's a quitter.
ダイエットはいつも三日坊主に終わる。
My diets always fizzle out after a few days.
I meant to exercise every day this year, but I gave up after just three days again.
I quit the diary I'd finally started after only three days.