DO NOT PUT LEMON JUICE ON YOUR SKIN!!
Lemon juice, when exposed to sunlight, is phototoxic and can lead to phytophotodermatitis. The results include burns, red rash, and hyperpigmentation (brown discolorations). Please do your research before you rub lemon juice all over your skin. The research all shows to not put lemon juice on your skin. When I mean research, look at actual studies. Not some random blog on the internet that anybody can write.
Even if you live in a cave, lemon juice is a bad idea. The short term good results you are seeing are from the citric acid in it, which is a type of AHA. However, the lemon juice pH level is not controlled. Yes, you need a low pH for exfoliation, but the pH in lemon juice is generally too low for the skin (in combination with a 5-10% average citric acid % in lemons). Over time, you will compromise your skin's acid mantle barrier, which creates a variety of skin issues. This type of damage can sneak up on you slowly and you can easily blame it on something else. You may not notice damage at first, and may think it works great - but over time the damage will add up. You are better off with a properly formulated AHA skincare product that has a controlled pH, amount of AHA, etc. There are some good cheap AHA products.
Why put something on your skin that has a HIGH RISK of causing long term skin damage for a short term gain? Use lemon juice for food, not for skin.