RE: I recently smacked my child...I feel horrible...
Take the guilt you are feeling right now, and use it for something productive - right NOW, while you are motivated to make a change, make a concrete plan for how you are going to keep your child safe. As usual, Katie had great suggestions.
When DS was little, I found that the sleep deprivation and isolation of being home with a baby who would *not* sleep anywhere but in my arms led to me feeling tired, then overwhelmed, and finally angry and resentful. For me, that's how depression presents. It was hard for me to admit that I was depressed, but I was more afraid of the person I was becoming (angry all the time, irrationally attributing motives to my helpless infant - i.e. "he's crying on purpose to drive me crazy!") I went on medication for six months, and it really helped.
Recently, as DS is entering the Terrible Twos, I'm finding that once again my patience with him is strained, and I'm more prone to lash out verbally. The return of the anger and resentment is my clue that the depression is coming back. What has been helpful for me was telling my DH and family that I was struggling with depression and needed some extra support and time to myself, reviewing some of my "gentle discipline" resources for strategies to use instead of yelling, and of course starting back on medication.
Honestly, I don't especially like being "dependant" on medication and I'm not happy about being labelled with Depression - but I know this is what I need to do right now, to be the best possible mother that I can be. I don't talk about it very often, I think because I'm embarrassed by it (believe me, I'm much more comfortable being a doctor than a patient!) but your message really resonated with me and I thought it might help you to know that you aren't alone.
Cindy, Mommy to DS 2/04 and DS 2/08