With anything in life, the number one issue is the commitment to do the task at hand. Fitness and the journey of fitness is on the same par as anything else in life. It even is so much more since the result is not just some extrinsic type of trophy or money but a much more intrinsic sense of self accomplishment. It along with learning is a long hard road paved with many ups and downs throughout the years.
As you begin your journey or are a 20-year veteran of strength and fitness you will understand the idea that nothing happens overnight or is realized fast. In the age of hyper gratification many people expect to see results over night. Prepare to be humbled and frustrated as this is not the case and almost anything and everything can be a setback that will happen at some point. Setting realistic expectations for yourself and goals designed to reach them in a great way to help increase consistency.
One thing to remember is that as you progress through your journey of fitness many of the tools of the trade so to speak are skills that are learned. If you want to keep these skills sharp you need to practice these skills often and with the expected intensity or desired level at which you wish to train these skills. All these efforts lead back to one simple trait which is consist performance of said skill or activity. At no point can you hone your skills if you are not consistent in your efforts to do so.
Many traits in life you will find are out of your control. One of the only things ever in your control is the ability to show up and preform the task. You can not change your genetics or really anything much about you anatomically. The only thing you can control is your effort and desire to get better every day.
Consistency is a novel concept but a very good life lesson to learn at any age. The great part about being consistent is that if you consistent then missing a few days or sessions it will not heavily or at all impact your progress in any form or fashion. This allows for greater leeway in life and will allow you to become a better version of yourself every day. Go out there and be consistent!