I'm glad to read on your profile, Kaylie, that you're staying away from the knives now -- and realized your ex was "really just an idiot"
-- hopefully things will be okay now!
But if you start feeling the urge to hurt yourself again, Linkat's post is good advice.
I had an episode in my life where I started cutting myself -- it was a mild case compared to others I'm sure, but definitely harmful -- when I was around 30. It just felt like physically hurting myself, if only mildly, took the attention away from my mental anguish. And that it was the only way to deal with it, since at that moment I felt (wrongly!) that few others would be willing to hear me out about what I was feeling. But it just made me feel more lost and alone.
A couple of things helped me, but of course not everyone will be as lucky to have these opportunities. I left the room I was living in to spend a couple of weeks with a family in the dunes. Lots of fresh air, beautiful surroundings, and I walked a lot -- physical exercise does help, as annoying as it can seem. Also helped by providing a quiet place, where I wrote a lot, away from home and reminders of heartache. Later a friend invited me to stay at her place - she wasn't one to talk about feelings but she was patient and didn't judge. Back home I also tried to get outside as much as possible, and forced myself to go meet friends even when I barely had the courage to do so. And I would really have benefited from counseling as well, which I unfortunately only got later on.
Also, the one time I accidentally cut deeper and there was a lot of blood, which left a scar for a good long while, scared me off. But obviously please don't wait for that to happen.
Not all of those things might help you. Or be possible for you. Or even be appropriate for your age -- please don't send your parents into a panic by just going off to stay with strangers somewhere.
But I'm just adding it to what the others have already said as things to think about if you ever feel like that again.