Don't beat yourself up about relapsing. It happens to many if not all of us. It happened to me and I remember how utterly despondent I felt afterwards. I wondered if I'd ever 'get' it.
The advice here to get straight back up and move forward is really good. Be aware of what led to your slip, and avoid that trigger/change your routine/look to extra support.
But never give up. Some times it takes many attempts, but it is really worth it I promise x