We were in this position. We decided (following advice from a health visitor) to take the pressure off, keep her in pull ups and take her to the toilet regularly, trying to work out a routine of when she needed to go and massively celebrating any success, even if it was just that she sat on the toilet for longer than a few seconds.
We took favourite books and little games to the toilet and found she would then sit for longer and became happier about going to the toilet. Once we were regularly getting her using the toilets (after about 3 months, but she had a lot of viruses during this time or it most likely would have been sooner!) we bit the bullet and put her in knickers. She had her first dry day about a week later, at around 3 and half.
A month on, she's having plenty of dry days and a few days where she has an accident or two, usually where she has left it too late to say she needs a wee! She isn't great at telling us she needs to go, so we tend to take her at intervals where we know she is likely to need to go and that works best for her.
So just to say that yes, have been in that situation but we are (hopefully) coming out the other end now.
Best of luck with it all, it's really brutal - easily the toughest parenting bit so far for us. Some kids just need a bit longer than others I think.