Boyfriend threatened to tell my parents if I do it again

I've been cutting (and burning recently) for almost 9 years, since I was 13, and only my boyfriend and the therapist I'm seeing at my university know about it. My parents don't really know that anything's wrong (as far as I know), they don't know I'm seeing a therapist. I don't want them to know because I don't want them to think it's their fault. Also, they tend to forget that I am a 21 year old woman who is capable of taking care of herself, and I don't want to give them more reason to think that.

The thing is, it's really stressful for my boyfriend. I hate having to burden him with all my problems, and seeing how upset he gets when I hurt myself. He's out of town this weekend, and last night I was feeling extremely upset about something. I knew that if I called him then he probably wouldn't have been able to answer. I wanted to swallow some IBU profen pills (NOT intending to kill myself, just to mess myself up a little), but decided against it and burned instead. I then called him a few hours later when I knew he could answer and told him what happened, including thinking about swallowing the pills. At first he said he would tell my parents next time I do it, but then made me promise to try to talk to/call him next time, and wait at least an hour before doing anything. I'm still worried he will tell them though. He said that when he changed his mind about telling them, he felt like he was doing something very wrong, very immoral and felt guilty. I don't know what to do. My parents can't know, but I don't know if can stop.

TL;DR: After a recent self harm episode, my boyfriend initially threatened to tell my parents about it, who are completely clueless that anything is wrong. He may change his mind and tell them, because he's very worried and I can't quit. Advice?