I don't feel 45 mentally, but my physical body often reminds me that I am in fact no longer in my 30's and a very long ways away from my 20's. I do not recover from my workouts as quickly. My metabolism has slowed down. I need to drain my bladder completely before I jump rope. My body parts are unable to defy the gravitational force. And my silver gray hair creeps through at a faster rate these days. None of these things are the end of the world, but are a gentle reminder that I need to adjust my training and nutrition accordingly.
Listening to your body can be one of your biggest challenges as an athlete. Your body is capable of so much more than your brain tricks you into thinking. However, this can also be your downfall because you overdo it. Learning to identify the boundaries between the two is a skill you need to master - when to push yourself and when to rest.
Learning to identify when you are physically taxed, mentally exhausted, being lazy, or just need a break either mentally or physically is key to getting the most from your training. The answer is not the same for any two people. This is why it's important to have a coach or a trainer who is willing to commit to you and your needs and not provide a 'cookie-cutter' training and nutrition plan. In exchange, you need to trust and be honest with your coach/trainer. Honesty with regards to your workouts and the food you are consuming are important factors. If you lie about what you are eating, you are cheating yourself and undermining your coach's efforts.
Trust the process. You didn't get out of shape overnight. You didn't gain weight overnight. Burning fat and building muscle takes time. A lot of time. Make the commitment. It's worth your money. It's worth your time. It's worth your effort. After all, it's only your LIFE, right?