6 weeks pp, always sad, thoughts about harming myself

I had my second baby 6 weeks ago and I also have a 2 year old. It's a lot harder than what I thought it was going to be. I feel sad and worried all the time. Sometimes I'm happy but not often. I keep having thoughts of harming myself, like getting into my car and just driving straight into a wall or cutting myself or something! I just feel like I'm not good enough and my kids would be better off without me