Hello sofiya and welcome to the forum.
First and foremost, well done for achieving what you’ve managed since the start of the year, both in terms of your weight loss and an increased levels of fitness.
You’re correct in stating that a lot of weight has been shed in just over a month, due in part to the daily calorie limit adhered towards but also due to the energy expended from stored fat. However, due to the calorie deficit introduced, an evolutionary trait of the human body is beginning to emerge.
Since you no longer possess the level of fat that you once did (excess energy), 1200Kcal has become an insufficient daily amount on which for your body to continue losing weight at the level that it has in recent weeks, so it’s holding on to the excess energy still possessed, hence why you’ve begun to plateau.
When you consider that the progress thus far has been achieved through a mixture of calorie restriction and exercise, some of your daily intake will have been expended during endeavour in the gym, meaning that your actually daily intake may have been as low as 900-1000Kcal on certain days.
In order to overcome your plateau, you need to increase your calorie intake as doing so will provide you with sufficient energy to continue exercising, while also allowing for the continued provision of a daily calorie deficit that’ll ensure weight continues to be lost.
To figure out how many calories are required to achieve both of the afore-mentioned aims, if you’ve not do so already, research BMR calculators since they’re largely accurate in determining BMR and TDEE, based upon your measurements (age/weight/height) and level of activity.
Having obtained your BMR, multiply that figure by 1.375 in order to ascertain your TDEE. Having obtained your TDEE, by introducing a daily calorie deficit of 400-500Kcal provided the deficit doesn’t take you excessively below your BMR, weight loss should be enjoyed once again, as the body is receiving sufficient calories to keep its metabolism firing, whilst allowing it to utilise the excess energy, stored in body fat, to cover the deficit created.
Granted, the level of weight loss enjoyed won’t be as extreme, but if you get your energy balance right, a weekly loss of between 1-3lbs certainly isn’t beyond the realms.
I hope that the above helps to demonstrate that it’s not necessarily all about the kind of food consumed; the key to successful and sustained weight loss rests upon the ability to achieve optimum balance between the energy consumed and the energy expended.
Concerning food, provided that you’re eliminating as many simple/refined carbohydrates from your diet as possible, opting for complex sources, in addition to consumption of lean protein and healthy fat, weight loss can be attained in the short-term and successfully sustained in the longer-term.
Concerning exercise, as your fitness improves, seek to introduce HIIT into your exercise schedule to maximise your level of fat loss. HIIT only needs to be performed twice a week in order to witness a noticeable improvement in body composition. Moreover, it will take your cardiovascular fitness to levels that you may have never experienced, something that’ll become wholly evident when participating in group exercise classes.