i am so sorry to hear about your situation..i know USMLE is very hard..specially for people like you and me who have trouble controlling their anxiety..but one thing is for sure..whatever decision you take be happy with it..
This is not end of the world or life..after 3 years of your dedication and hard work even if you quit USMLE , it doesnt make you any less of a doctor. Remember 1 thing, we are and always will be among the top 1% of the population. We can have a bad experience but who doesnt. If you discuss your situation with people who could not get into medicine you will realise how noble our profession is and how fortunate we are..and btw these are just exams and let me tell you CS is a lame exam , it doesnt judge your clinical skills in any way..And passing and failing in this exam involves a lot of luck..its not the end..if you have a dream you will be able to fulfill it as long as you do not quit. USMLE is not for competitive people only, sometimes even those who were very average in their med school do extremely well and vice versa. And life isnt about winning a competition every time..think about it..also talk to your friends and family, they'll always be your side no matter what.