How can I get my girlfriends trust back after I lied to her?

it was halloween and i got high i told my girlfriend and got mad at me so i told her i would never do that again, so then christmas came around i got high but never told her february came around and got high again i never told her because i didnt want her to get mad and upset with me so a couple of days ago i came clean and told her about it (i lied to her about it for 6 months and weve been dating for 8) now we had a giant fight and now she doesnt trust me i love her to much to let her go but i dont know if im able to fix things i promised to her and myself i would never lie to her again but i know its not enough and she says she doesnt know how or if she can ever trust me again i want to be with her and i want to grow old with her shes my life shes my love and i dont know what i would do with out her but she needs to regain her trust back in me but i dont know how i can do that or how i can show her that she can trust me again other than not lieing showing her my love and telling her the truth at all times. please help :(