Harvs honey, you're not alone. My 3.8months old is DRIVNG ME NUTS. Again.
From maybe 18 months - about 3 was rough - she would hit, bite, scream etc. I had bruises all over me. She was also an uber clingon. I could never do anything without her getting upset. Even going to the loo would result in a breakdown.
Some days I barely made it through. Her sleep was also crap, so I was exhausted and struggled. A lot. She nearly broke me. If it wasn't for my friends, I wouldn't have made it through.
Anyway, different things worked for her at different times ages. When she was in her clingy phase, we did cuddles and she was fine. Then she got less clingy and the biting got worse. Trying to cuddle her made it worse. She would just scream at me and hit and bite. So after trying to parent gently, diverting her etc etc and getting nowhere, I put her in her room. I tried not to get worked up, would just put her in there and say "we don't hit, when you can calm down, you can come out and we will have cuddles". It took doing it multiple times for about a week (she's a tough nut) before she realised her strategy wasn't working.
So we finally got her out of all the hitting/biting and things were nice for a few months. But now? She has the attitude of a teenager. Yelling at me, stomping her foot, sulking and back to tantruming if she doesn't get her way. Cuddles are working again, but it takes a good 15 minutes to calm her down, and 2 minutes later she will cry again. Repeat, repeat, repeat. I don't mind the cuddles, but an hour over one incident is exhausting for both of us. And it can happen 6 times a day. And I don't always do so well towards the end of a long day.
Anyway, after all the novel, my picking battles is always to challenge if she is being violent/mean to anyone. I don't fight if she won't eat, I just put a timer on and if she won't come, it goes to the chickens. I don't fight if I am trying to get her out the door or get my DS to school, and I rarely fight at bedtime. So for me it's more about timing and the severity of the battle. I will give in if it means being late to school, or having a later bedtime because I then have to settle her down.
With the hitting, I tell her her friends won't like her if she hits. That it hurts Mummy and hitting is not nice to do. I have also given her a light tap on the hand to show her it's not nice (and I admit I smacked her when she kicked me hard in the stomach which I'm not proud of). I find giving her more independence has made things a little better - she chooses her own clothes, dresses herself etc etc. I'm sure the majority of her behaviour is frustration.
I also take favourite things away - toys, iPad etc if she is being really naughty and it does help for a day or two. She is slowly getting better, but it's hard. Yesterday I gave her her lunch, which was a plate of all the things she had asked for and helped me make, said "here you go" and she screamed at me "mummy just stop it". 5 minutes later she said sorry and gave me a cuddle, but I am sick of being screamed at because I've done things for her 😪 At the moment, I'm just trying not to engage her and just say "I love you" when she does it. Not sure if it's working, but one can hope!
This parenting gig is hard. And I have support. Be gentle with yourself. None of us is perfect all the time, we just do the best we can.