Hi OP, I feel you, despite repeated attempts from the age of 2, my eldest wasn't toilet trained until Christmas before he started primary school. Some things that helped us:
Lots of support from the health visitor
Continence team referral (from age 4 in our area)
Reading the ERIC website and joining some of their webinars
Treatment for constipation (constipation can cause wee accidents)
Scheduled toileting (going every hour to start with, the increasing the gaps) DS didn't self initiate toilet trips for ages.
Training pants - disposable and reusable both helped
Ditching the potty and going straight to the toilet
Binning the Oh Crap potty training book
Realizing that some kids don't 'get the hang of it in a week when they are ready'
... basically keep asking nursery/HV/GP/ERIC for help. You really don't have to battle this on your own.