How do i explain to my long term(2year) boyfriend that if i cut its not a reflection on how good of a man/boyfriend he is?

So we are Vary happy and devoted happy partners, i had a cutting problem for a long time, but when we started dating i was just starting to get better, weed helped a lot for my insomnia witch in turn helped my self harm... I had been almost a year then i slipped up and cut my self again. He said he would away love me and help me though it... almost another year goes by, then i get stressed out, I start having those thought of hurting my self, i don't say anything i don't want him to worrier, "im fine rely i can do this" and then when i'm alone, those kind of thoughts start racing though my head, then i self harm.I'm 21 years old, when i was 12 i never thought i would be still doing this. i though i was over it, i thought I was better. this addicting "coping method" is getting in the way of my relationship! i love him so much and the disappointment and fear in his eyes when i have to tell him i slipped up again, brakes my hart the guilt of what ive done is unbearable.naturally he hates to see me do that to my self, but he takes it on him self, he thinks I did it because he is not good enough and dose not make me happy, or does not take good enough care of me i've tried to explain to him that the to are separate i don't cut when i'm unhappy i cut when im suffering emotional pain and that is never caused by him, he is the best man i have ever known, i want to stop this cycle for him. i don't want this problem any more, i don't want to have to face him and tell him what ive don.