Should i go n fight or just run as a coward forever?

i have just had enough of the world around me. for 15 years i have suppressed my anger inside me. people just don't stop bulling me around and mock me. i know many of u are out there who's in the condition like me. but let me tell u what happened today. a kid not a kid but he's like 14-15 yrs and i m 18 yrs. he came in front me and said ooo little bro do u know me?? if u do give me some money or ur in trouble. i mean come on who does he think he is. and who cares who he is... i was just replying like no i don't have any. i felt like **** getting bullied by a junior and not being able to do anything. i m not afraid of fighting but i don't know y i just seem like a coward not being able to do anything. to be honest my left feet and heart beat shakes like hell when i m in situation like this. what should i do??