For centuries people have been making New Year’s resolutions, endeavouring to be a different, better person from 1 January. I’ve done this, and I have failed at every single one.
Then I stopped. I stopped making New Year’s resolutions. I started and stopped thins on random days. Those are the ones I stuck with.
So maybe stop waiting for Monday, or 1 January. Do it on a Wednesday. Do it in the middle of April.
If it’s something you really care about, you’ll want to start as soon as possible, and you’ll be more likely to stick with it.