Why do I get suicidal when life gets stressful?

Quick question, for some reason when life gets stressful I start getting suicidal. Like the other day, my ex was kicking our daughter out of his house. He didn't even care if that meant that she had to live on the streets. She's 24 and autistic and can't work. For a while I did everything I could to help her. Nothing was working. Pretty soon I started thinking if I can't help my kid what kind of parent am I and should I still be around. Yes, I love my daughter and I think that's what pulls me back from the edge but why does my brain go to killing myself. By the way she now lives in the same house that I'm renting a room. Fortunately my landlady is also a good friend and helped me out. Of course as soon as everything was settled, I was fine. It's very confusing.