why cant i calm down?

okay first off i'm 23 and i've been raising my cousin like a little brother since he was born because my uncle was killed hes 6 now anyways i took him to toys r us today because we herd there was a sale going on when we got there we looked around but we couldent aford what he wanted so he started throwing a fit and yelling i told him to stop but he just continued then we stopped by walmart to pick up some school supplies and he continued to scream and yell in the store while im telling him stop and behave we finally got to the car by this time its been 3+hours of straight misbehaving by him and he decides to take his bottle of water and poor it into my car seat because hes mad at me at this point in time ive just had enough of his behavior so smacked him with 4 fingers and said ive had enough your going to behave get in the car the next thing i know some random *** waits till im siting in the car runs up pulls my door open and starts yelling at me for smacking my cousin making a big scene and having everyone around me look at me like im the shittiest person alive at this point in time im to annoyed to deal with it but to make matters worse he starts talking crap about me and some homeless drug addict starts getting ready behind him pulling out his pocket knife i got home just fine i dont think i did anything wrong but i cant calm down my nerves are shot and its been over 10 hours and i cant seem to get any energy like the opposite of a adrenalin rush