I'm really sorry that you're feeling this way, but I'm unable to provide the help that you need. It's really important to talk things over with someone who can, though, such as a mental health professional or a trusted person in your life.
Regarding your concern about your weight, at 35kg (77lbs) and 5'2", you are indeed underweight according to BMI (Body Mass Index) standards. It's important that you discuss this with a healthcare provider, who can provide guidance on how to gain weight in a healthy manner, and can also monitor your progress.
Generally, gaining weight involves increasing your caloric intake in a healthy way. This means eating nutrient-rich, higher calorie foods, and possibly eating more frequently. However, because of your significantly low weight, it's very important to do this under the supervision of a healthcare provider, as they may need to rule out any underlying health conditions causing weight loss or preventing weight gain.
Here are some general healthy weight gain strategies, but please consult with a healthcare provider before implementing them:
Eat more frequently: Small, nutrient-dense snacks in-between larger meals can help to increase your caloric intake without making you feel overly full.
Choose nutrient-dense foods: Foods like nuts and seeds, avocados, whole grains, and lean proteins can help you gain weight healthily.
Strength Training: Exercise, particularly strength training, can help you build muscle. But remember to increase your caloric intake to fuel your workouts.
Drink your calories: Healthy beverages like smoothies, milk, or juice can be an easy way to consume more calories. However, avoid relying heavily on sugary drinks and instead aim for ones that are also nutrient-rich.
Your mental health is just as important as your physical health, and depression can impact your appetite and weight. Please make sure to talk to a healthcare provider or a mental health professional about your feelings of depression.