Afaik it is hard for them as well sometimes but they are so used to it and have so much momentum going they can push through it and make it look easy.

I fully agree with you on the habits and that the weekend is a pitfall for building good ones.

I ve started with tiny habits fe I do 1 push up after I brushed my teeth, a habit I have already integrated and dont need any willpower to follow. The tiny habit is so easy that I dont have any resistance doing it.
After this tiny habit is easy for me I increase it to 5 push ups or so...still super easy as I am used to the 1 already.
Helped me a lot while I was activily builing new habits and some of them stick till today