I bleached my hair a few months ago now its constantly falling out. Now I've probably lost about 2/3 of my hair. What can I do?

I agree with Dr. Donovan that you need to see a hair loss expert. They will determine if the hair bleaching has damaged the scalp,  your hair is breaking or there is another etiology. Treatment for each of these causes differs.

After bleaching your hair follicles that were already damaged (miniaturizing) from the effects of DHT, may not return. At your age this should be minimal. The hair follicles which were not damaged by DHT should return healthy in approximately 2 more months. In the meantime, you should see an experienced hair surgeon for an evaluation - preferably a member of ISHRS ( International Society of Hair Restoration Surgery - ISHRS.org

I'd strongly recommend you see a physician who understand hair loss. The bleaching could be related (and even a cause) but other factors need to be ruled out. Please see a physician as soon as you are able so that he or she can review your whole story and examine your scalp up close.

Bleaching hair Can, if Performed improperly, can cause permanent hair loss by damaging the hair at it’s root at the scalp level. See a dermatologist to find out if that happened to you.