Oh, you poor thing, OP, it sounds like you were absolutely at the end of your tether.
I too sometimes hit things when I'm angry (usually my hand/arm against the wall, sometimes my fist against my body / head) -- it upsets me that I do it; I've been getting some counselling which has helped a lot.
I've smacked my 4 year old maybe 5 times in the whole of her life -- always when I've snapped and lost my temper. Never hard.
Like you, I feel terrible afterwards.
Go easy on yourself. I wouldn't bring it up with her tomorrow unless she talks about it first -- otherwise you risk making it a bigger deal to her than it actually is (from her perspective).
When my DD has brought up "when Mummy smacked my leg..." or something, I'll say that I'm sorry I did it, it was wrong of me, and that "we don't hit in our house".
(She and DS, age 2, hit me and one another sometimes, so we try to frame this in a context of a family rule to not hit.)
I think there's a vast difference between a one-off smack and using smacking / spanking as a disciplinary tool. You sound like a loving mum going through a very difficult day.
Give her lots of cuddles tomorrow, try to have some nice times together, and do get all the support and love you can have for yourself. I completely understand how the hitting yourself is a pressure release it is for me too but you can get on top of it and find healthier ways.
I also totally get how incredibly tough it can be to be denied some time to yourself when you desperately need it and have been looking forward to it.
Something I've found REALLY helpful in recent weeks is the idea of "self-compassion" -- if you Google it, you'll find some practical tips.
Sending hugs.