Wow. Three years is AMAZING. You should be so proud of that. I know that slip-ups are hard, but they happen to the best of us, and they are a necessary part of recovery. Maybe think of it this way: you've only cut once in the last three years.

Try not to stress too much about telling your therapist. He's heard things like this a hundred times, and while he might be disappointed for you because you're going through a hard time, he won't be disappointed in you. And like you said, he should stay updated on how you're doing. Good luck!