I have been a personal trainer for a lot of years. A lot. Enough that I've created workouts for longer than I've known my 11 year old. I love making programs for other people. Seeing what they want, what they need and creating a plan that I know with certainty will get them there. I really enjoy it.
You'd think that going to the gym and kicking my own butt would be easy. It's not. When I go to the gym without a plan, I do the exercises that I enjoy. Are they the exercises my body really needs? Probably not. Case in point, I don't enjoy lifting on leg day, but I really want to build muscle on my legs. Give me a choice and I will gladly do 20 sets of back exercises, or 10 minutes of planks before I will give you another lunge. I am working on this, but am not there yet.
It is best to go to the gym with a plan, ideally a printed out workout or something on your phone that you can look at and check off what you've done. Currently I'm testing out some of my own workouts, making sure they will do what I say they are going to do. Sometimes as a trainer I like trying out other trainers' programs for my daily workout. It's nice to get variety in, and I love getting new exercise ideas. I will let another qualified individual boss me around and I will do whatever they say. On paper anyway. I have been doing James Wilson's 12 week program off and on this past year and love it. I started weeks 13 - 24 about 2 weeks ago and am also adding in a workout from Bret Contreras' book Strong Curves which I'm loving. I felt like I needed to add some leg work to what I've been doing. So far, I'm loving it and loving my new hammie and glute soreness! Look for a book review on Strong Curves coming soon.