How can i overcome a that is nothing but a replay in my mind?

its been almost three years and yet i still have nightmares about it. i mean it is just really getting to me severly. i lied to my doctor when he asked if i was ever d. i had to he would have told my mom and she cant know. im on depression medications and it is not helping me any. the night just replays and i wake up with my heart pounding from dreams about it. i feel like it is my fault for not doing more but yet i dont know what more i could have done. i feel like i should not have went walking that night and in my mind all the time it replays, the sounds i heard the way i felt that night from beginning to end, the way i saw things after it was done. i mean everything just looked idk gloomy like and the tree. all i could see when it was happening was a big tree like covering me so noone could see me. i dont know how to get over these memories or to get that night out of my memory period i mean. i just recently told one person and she wont tell anyone that is why i told her. but i know it affects me with guys i dont trust easy i wont be alone with them i am terrified to be home alone. even tho it didnt happen at my home i just cant be alone at night. my boyfriend wants to know what is making me act the way i do because i have even noticed im depressed and just not the same peppy happy girl i used to be. i have scars on my arms and instead of cutting i snap my arm with a rubber band. he asks why but i cant tell him i mean hes been my bf for a month. should i tell him about it. and what can i do to erase this memory. im tired of it. i feel like i am reliving it. i can feel the pain of everything he did and i see everything i saw that night. i dont know what to do. i can see how i looked when i came home. my clothes were ripped i lost my favorite bracelet that night its still there i havent went back there, my hair was completely messed up. i know that i was soaked in blood. i dont know how to forget it. what should i do. im scared to tell my boyfriend because i dont want him to break up with me and im scared he will think its a stupid reason not to trust him and i want these memories gone