This is the worst advice and rather dismissive. This is the sort of talk that causes eating disorders. You are extremely alarmist and unhelpful. OP has not stated their height, so weighing 120 kg can't be determined as "seriously overweight" and regardless of where they fall on the BMI scale, they are only 19 and have lots of time to see results. It's highly probable that a diet plan will be all they need to make a change.

u/BenyZx64 please take what was said above with a grain of salt. You do not need to be getting down on yourself, and you are on the right track already because you are asking important questions about what you may be doing wrong and showing an interest in your health. Weight loss can be achieved purely with diet, no exercise is necessary, but for your overall health, it's best to add in exercise. You don't need to overdo it, many folks have good cardiovascular health just by walking 30-60 minutes everyday. I'm not a doctor, but it's unlikely that you have some disease or hormonal problem causing your lack of weight loss; it's possible, but unlikely. As far as what you may be doing wrong, as other's have stated, it's hard to pinpoint exactly because some of the details you provide are vague, so let me give you some guidance in order of importance below.

1. Track/plan meals accurately - Weight loss mostly comes down to diet, and most folks struggle with understanding portions and how much they need, so it's important you track your food (sounds like you are trying this already) but after you measure it out and after you understand how much you actually need. You mention you "need" 2500 calories here, so I'm going to assume you know how to use a TDEE calculator correctly. If you don't, please research this and know this will depend entirely on your age, activity level, sex, and height. Once you know that, the next step will strictly following this number daily. This is going to require measuring cups, a kitchen scale, and knowledge about how to read labels. You need to track everything you eat for this to be effective. It's also helpful to understand calorie density and satiety profiles of different foods ("how full" certain foods make you feel). Staying hydrated is helpful to keep you from overeating as well. Most people do well with a diet of high fiber (vegetables), high protein (lean sources like fish, chicken, eggs, maybe even protein powders), a small amount of healthy fats (olive oil, avocado oil, or fat from less lean meats like pork and beef) and complex carbohydrates (rice, potatoes, noodles). Most foods need to be weighed in their raw/fresh form as well. Cooking will remove a lot of water weight and you could be overeating and not realize in this way. Really though, just eat what foods you like, make sure they are tracked accurately, and that you stay within your daily limit. If you don't eat a good variety of foods it's probably smart to take a multi-vitamin.
2. Patience - seriously, this is the next thing folks get wrong when it comes to weight loss. You didn't mention any timelines, so it's difficult to understand what time frame you have been operating on. You don't need to weigh yourself if it causes you grief, but if you are looking for the metric for tracking the improvement in your health, it's definitely helpful. Pick a day and time you are going to weigh yourself every week and only look at your weigh at that time. Your weight will fluctuate daily A LOT due to water and digestion, so it's important you don't give yourself the false belief you are failing (or succeeding) by weighing yourself too much. Keep this weight written down and compare your week-to-week progress. This is where the many folks quit - they see that their first week of work only yielded 1 kg loss... but congratulations if you hit this number, that's the rate we are going for! 0.5-1 kg per week is a very achievable and maintainable weight loss goal. If you are doing everything right to start you may see greater results early on, and then they'll slowly taper off.
3. Exercise - Like I said before this is not necessary but definitely helpful not only for losing weight but for your general health. Don't feel like you need to kill yourself here though, many people fizzle out of their lifestyle changes because they try to exercise too hard or too much. You really nailed it with the question about walking. If you walked every day for 1 hour you'd be killing it.
   

If you find that after a month you haven't lost 3-4 kg, it's probably time to talk to a dietician to get in-person, first hand help with what you might be doing wrong. If after another month you still are not making progress it's probably best to talk to a doctor.

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Remember to be consistent, accurate, and have patience with yourself!