How can I tell my girlfriend about my relapse?

So about 5 days ago I relapsed and I still haven't told my girlfriend. She knows that I started self harming about 2 weeks ago. I was 6 days clean but then i relapsed. I don't know how to tell her. She will be disappointed and upset (reasonably). I also don't know if i even SHOULD tell her. She isn't my therapist (i dont have one) and it's not her job to deal with this kind of stuff. I know she loves me and cares for me but I haven't been able to tell her about my relapse because I don't want this to be her burden too. I really do feel like i can recover without her help, but she specifically told me to tell her if it happened again. I really dont know what to do.

Some advice would be much appreciated!