How can i convince my parents i'm not smoking?

back in the summer i was smoking alot of weed (im 14 btw) and eventually my parents caught me in late august they finally caught me and theyve been very suspicious of eveything ive been doing since then and i really have quit but the other day my mom saw some black stuff around my toilet (my mom is ******* stupid) and thought it was ashes and i told her that it wasnt and she didnt believe me cause "she knows what shes talking about" and i got grounded for that and just now my mom and i got into a bad arguement and i know that only makes things worse but im starting to go crazy now i just now almost had a mental breakdown and i just dont know what to do now so im asking you guys if u have any personal experience with this kind of situation then please help me cause im really about to hit her in the ******* face