Start with a week, and give yourself a “cheat” day. Knowing there’s a cheat day coming helps with the initial transition.

It changes “won’t” to “can’t”. It’s not that you just won’t break your diet, you can’t, because it’s not cheat day. Then on cheat day, have the thing and don’t feel bad about it.

After a while, you’ll find you don’t need/want to cheat as much, and can delay cheat day for longer periods.