Even if you already know how to become a personal trainer and are great at your job, it's hard to a living. Training in a gym as your only source of revenue isn't the way to get it. Many of the limitations of training in-person actually REDUCE your effectiveness as a trainer. You're also stuck in one place. Want to travel? That's fine, but you won't get paid.
Not only do you make less money in a gym, you're chained to one place, AND you're not even helping your clients as much as you should be.
Think about it: does every client really need exactly 30 minutes or 1 hour to train? Do you want to continue to work early mornings and late nights just to make ends meet? Do you want to travel? Or spend more time with your loved ones?
Online personal training is the answer, either as a primary vocation or as a supplement to your in-person training business. And if you own a gym, you should be including an online training component as a supplemental income stream. Building an online clientele solves every major problem skilled trainers face.
The Internet and available software are there to help you scale your efforts and develop multiple streams of income (some passive) through online training. But if you don't have the proper systems, you'll end up exactly the way you were before: trading your time directly for money. The difference is that now you'll be stuck in front of a computer and not moving around in a gym.
You must have killer systems in place.