Why do I yell?

Tonight was awful.  I lost my shit completely and now I'm sitting here, once again, wondering what the hell is wrong with me.

This is how it went.  10 year old spills milk, step-dad pulls paper towel out of my hand as if I'm too stupid to clean it up and goes to do it.  This pisses me off and I make a comment.  He says "I'm just being a parent".  I respond "I was fully capable and you just took it".  Mood change instantly.  10 year old makes smart ass comment to step-dad.  Step dad yells at 10 year old for talking rudely.  Not just one sentence though.  He goes at him about respect until tears form.  I believe this attack is because I pissed off step dad.  I jump in and tell him to back off...enough.  He says "yeah? you try dealing with him all day", enter my smart ass comment "I would but I'm busy working all day to pay the bills around here".  Full out fight now. 

Remember, this is at the dinner table, with 3 kids.  THIS is NOT who we are.  THIS is NOT our family.  At least it didn't used to be.  I call him a 200 lb. 6 foot 2 bully.  He leaves the room mumbling about my newfound level of crazy.  I continue to call him a jerk from the kitchen. 

Then it hits, and hard.  The 10 year old points out that 13 year old step-daughter is crying.  Reality check big time.  I go to her and say I'm so sorry.  I'm so sorry I yelled like that.  It's too late though.  It's now a memory.  A terrible memory I made for her.  I'm sure all she wants right now is her mom.  Not her crazy ass step mom.

So I ask again, what's wrong with me????  When did I become this person.  This monster that screams at the dinner table?  When did my spouse become so angry.....he never yells, like NEVER.

I just don't know what to do anymore.  The tension in this household is through the roof and it seems like me and my partner don't even like each other lately.  I'm not basing that on this one spat.  It's been weeks of nasty under breath comments and quick snapping about silly things.

I want to stop being this person.  I want to say I'll never yell again.  I'll handle this better.  I don't even believe myself though, so how could I expect them to?  I will do everything I can though to be damn sure I never let myself make them cry again.  I never want my children or step-children to be scared or sad with their own parents.

Wow, things are so messed up.  So terribly messed up. 

I only write this here because I don't know any one who reads this and if judgement is passed on me than so be it.  I needed to vent though, I had to get this out because I feel like I'm falling apart.  I try so hard to be positive, to be hopeful, to be happy.  I can't make it real though lately. 

I think all of this change is just too hard for the family.  I'm sure tomorrow will be better, I know it will.  Tonight though, is bad.