Don't miss twice!
We all always try to have good habits and wish to follow them for a long time.
But we miss it 1 day and never get back to it because you broke the rhythm you had.
There's a concept which I started following recently which is from the book ATOMIC HABITS - by James Clear.
And the concept goes like this don't miss twice even when you don't feel like doing it the next day - do it anyway after you missed the first day.
What happens when we miss 2 days continuously is we break our rhythm and get back to old habits.
As you already know it takes 21 days to form any good habit.
Be Consistent and Disciplined :)