This is the 5:2 diet website which is habituated by people trying to lose weight and/or become fitter/healthier.
None of us are experts, nor is anyone qualified to give advice on long term fasting apart from saying NO.

The best thing you can do to improve the appearance of your skin bearing in mind that at your age, hormones will be affecting it, is look at your diet and cut out all the unhealthy stuff, get out in the fresh air as often as possible and do some regular exercise.

At your age fasting is not something you should be considering especially as you are not overweight. The body does not stop growing until you are 25 so you are wise to be concerned about affecting such things by considering taking drastic measures that are not necessary and most likely will not help address your concerns.
Your body is a very efficient mechanism which is perfectly capable of getting rid of any substances that it doesn’t want or need and detoxifying is completely unnecessary as nothing you can do or take will be as efficient as your liver, kidneys etc in ridding your body of toxins.
Many processed foods and additives can make you feel sluggish. tired and generally out of sorts which is why I’ve suggested you look at your diet.

You say you have pain but do not specify. I suggest you make an appointment to see your doctor in order to discuss all of your concerns.

Good luck.