Getting mojo back has more to do with accepting your present age than it is trying to recapture your youth. If you strive to become the best you can be at your present age, you’re already halfway towards your goal.
Figure out when you last felt good about yourself, inspired and happy. Has it been a year? Five years? 20 years? If your last happy moment was in high school, start by being honest with yourself about this. If the last time you felt good about yourself was when you had your second child or when you were married, do an assessment of why you aren't happy now.