Life is not one big happy joy ride, everyone goes through trials and experiences times of discouragement and disappointment. We must expect such times as well as better times of joy, satisfaction, and achievement. However, there is no need to fear such trials, because when they come, Jesus will be there, ready to help us deal with them.
Consider cutting out sugar from your diet. When we're young, we can safely eat those pastries, ice cream sundaes so long as we work off the excess calories. As we age, however, our metabolism changes and we must work to keep fat from accumulating.