This is, unfortunately pretty common. I was literally just about to post about my experience with the same thing when I saw this! People think I can't be depressed because I'm "not as bad as that one time", and can make it to school or work and have friends over still and I'm not cutting anymore. That doesn't mean s**t.

The only way to let them know is to tell them. Still they might not believe you (I had a friend tell me that it was hard to take me seriously because I talk about suicide like the weather), but they might.

Curiosity77, I had a friend go through the same thing, and she hasn't been able to get help since.