You must always force yourself on, performing whatever your coach tells you to do. Swimmers suffer some of the most pain imaginable, for their whole body is consistently subjected to the wrath of the pool. After some training sessions your body is so beat up that you walk into your living room and drop dead on the floor. No muscle group escapes use in swimming. Hence the mental toughness and desire come into play, for you must defeat the mind games you begin to play with yourself.
Swimming is a cruel sport.
You can train with all your heart and soul for a year and, just before the important meet, fall ill. Or at that one meet you can have a single poor performance due to a large amount of details that could go wrong. Or you can get a devastating injury, the most common being one to the shoulder. To come back from injury is a difficult journey in itself, and one that defines a person's character. Talent is just as great a factor in swimming as it is in anything else, and those that have it do not always realize the potency of their gift.