Working out isn't only about being and looking fit, it's about a health journey, about cutting processed foods, learning to have a better relationship with yourself and with your body and being happy even if you are 5 pounds heavier than you thought.
Your body goes through such a change that it also makes you change mentally, but there are times in which this can bring the opposite result and that when you shouldn't be working out, at least until you understand why it's not the best for you.
You tend to obsess and not in a good way. The moment you set a goal and obsess over it, it's time to stop, when you get mad because your progress isn't as good as you thought, when you spend more time on machines than you should, when you eat less than you should, that's when you need to stop.
You are doing it for someone else. Maybe you want to impress someone, maybe you think this person is going to think you are more beautiful because you look thinner, when in reality you should be doing it because you want take care of your body, you want to be healthy not because you want to attract someone else. If they think that way, they aren't worthy.
You've been told you are "fat" or "lazy". I've been there, it hurts a lot when someone starts commenting on your size and the way they think you should look, but everytime someone tells you something like that you should brush'em off and tell them to worry about their life. No one should ever dictate the way you look.
You have injuries or have suffered from eating disorders. This is a controversial one but goes hand on hand with the first one. In my case, if I were to go into working out right after I recovered from an eating disorder I would've obsessed about it and overworked myself. Learn to know your body and what it needs, talk with and specialist, spend some time resting, set a realistic goal and slowly work yourself towards that goal, but don't rush it.
You are comparing yourself to others. Another one I'm guilty of. I've compared my body to others when my frame isn't as small, I have curves, I will never ever look like some of them because we have different body types and that's okay. You'll look amazing, no matter how many flaws you think you have. Bodies are amazing and should be loved.
You think a restrict diet and killer workouts are going to help you reach your goal, because you'll be disappointed when it doesn't work the way you wanted or when you reach a slump and the scale isn't going down anymore. You have to learn to have a good relationship with food, eat carbs and fats, they are amazing and necessary (as long as it's in moderation and you are also adding fiber, protein, etc.). You have to learn that cardio isn't going to do everything for you, weights are good for your muscles and rest is your body's best friend.
Fitness journeys are hard, I stopped weighing myself about two years ago, I stopped obsessing about having 7 workouts a week about a year ago and I started loving my body more and more about 6 months ago. I eat in measure, I don't avoid sugar, carbs or anything that has been victimized, because that's what works for me right now. There's a lot more I would want to work on and I'll do it, but I won't punish myself if I don't look the way I wanted in 15 weeks.
Learn to love what's around you, learn to love your body and everyone will turn to look at you.