21 days is a minimum, only applicable to the most easily formed habits

Grabbing a fruit after dinner was relatively simple to achieve. I forgot sometimes in the beginning, but by the end of the month, it became instinctual. Even still, it took me about 25 days to develop this routine. Other simple habits that may be formed in 3-4 weeks are things like drinking water with a meal, taking shoes off before entering the house, placing the car keys on the key hook, etc.

More difficult habits may take months to develop.

It took me about 6 months to stop biting my nails. I typically did this when studying, reading, or watching TV. I tried to replace it with other habits, such as fiddling with my hair tie or a pen/pencil. In this case, "21 days" was a horribly deceiving lie. I realized this was closer to the truth: most habits that we wish to employ in our lives are challenging and drastically reform our daily ingrained routines and tendencies. Things like working out, meditating, flossing, drinking eight cups of water a day, etc., require a lot more patience and effort to achieve.

Mistakes do not mean all progress is lost, and progress is not linear, either.

There were many instances in which I accidentally bit my nails, forgot a fruit, missed a day of Duolingo, and missed a day of exercise. Humans are not perfect, and when attempting to form a habit, you will most certainly make mistakes and miss an opportunity for development every now and then. However, what will determine your success is your ability to rebound and find strategies to continue where you left off.

It doesn't gradually become easier as you progress, either. There will be days several weeks or months into your progress where you find it more difficult to employ the new habit than it was in the beginning few days. This should not be a source of discouragement -- everyone develops at their own pace, and it's not a perfectly straight path to success.