If it hasn't closed up by now it sounds as if there is an underlying problem. It needs a proper clean out and any necrotic or infected tissue cut away, disinfected, and the wound closed up before any infection gets into her blood and you lose her
Only the vet can do this properly, she may need to be sedated if she is not easily handled. I agree with the others, a vet visit is now unavoidable.
Regardless of the cause of the initial injury, this is not healing as it should and there will be a reason for that. Putting off professional medical help will only serve to make her recovery more prolonged and less likely, and more expensive for you in the log run.
This looks painful for her and you need to get it sorted. I appreciate of course that vet appointments aren't easy or cheap over a week-end but can you get Millie to a vet today?
Doesn't look like a cat bite to me. Thought they were usually two hole punctures? I'm no vet though and she needs to see a vet
Tufty only ever manages to get one hole in me when he bites, Rosella
it's down to the angle of attack. Bless his shiney white fangs.