1 - Not following a diet - Any competent personal trainer will tell you this: a radical body transformation is IMPOSSIBLE without a proper diet. Mind you, exercise is a key in losing weight, putting on some muscles. But doing so drinking every day, eating pies like there is no tomorrow? Forget about it.
2 - Ego lifting - How many times you have seen somebody failing an exercise spectacularly? Learn the basics: is absolutely USELESS to lift 100+ kg if you are doing it the wrong way. Injuries. Wrong muscles used. Fatigue. Really, if your personal trainer tells you you are doing something wrong, LISTEN TO HIM and STOP EGO LIFTING. Go back to 20, 30, 40 kg and persist until you are able to lift a bigger weight PROPERLY.
3 - Quitting to early - As everything in this hard and harsh life, changing your body is difficult, expensive, hard and, read this two or three times, REQUIRES TIME. You can’t invert a lifestyle of bad habits with 2 days in the gym. Putting on some decent muscles? It can require 2 or 3 months. Losing 10 kg? It can take months. Plus, it really depends by a lot of factors how long you will take to get where you want. Sleep. Stress. Genetics. Really, there are a lot of things that can slow down your progression. Be consistent, don’t lose hope and keep hitting the gym, months later, you will be thankful for doing so.
4 - Comparing yourself to others
This is unavoidable but still.
“Oh no, my chest is so small compared to X!”
“Damn, I will never have a decent back like Y!”
“Why I have to train if I am still fatter than Z?”
Ignore what other people are doing. Positive competition is a thing but feeling sad because you are not in shape like influencer X (who maybe even modifies his pics online with the AI or simply can afford an entire equipe of nutritionists, medics and so on) will destroy your motivation to train. Keep your eyes on YOUR journey, not on other people’s. You are not going their way.