Just wanted to tell you, you are so not alone. I was smacked as a child and after 3 years it is still instinct for me to smack (worse still, it is out of anger/frustration ) Usually when Im at the end of my tether, but I still do it. I wish I had asked for help at the very begining, like you have. So good on you!! It is hard to admit and ask for help, but you have taken the first step. It is extremely difficult to break this habit. And I beat myself up every single day for what I put my baby through.
Im working on it, very slowly getting better. But it is HARD.
I hope it is easier for you because you caught it early, rather than let it go for a year or so before trying to fix it when it got really bad.

One thing I will say is try not to replace it with yelling. I did (not intentionally) now I have 2 bad habits to break