Help! Potty training stand off!

Hi  everyone,I need some advice. My daughter is just over 3, she is very bright and loving - but stubborn as hell!My husband I recently started trying to potty train - we put it off for a while as she wasn't really showing any signs of readiness plus she often withheld poo and had some issues in that area, so we were really nervous about pushing anything to do with toilet training in case that flared up again. Anyway, she seems to be OK on that front for now (touch wood!) so we have been trying to really go for it re: toilet training, at least for wees.Anyway, for aaaaages now we have had a star chart up in the kitchen, which we started initially when she was withholding poos, as it was advised to us to start one for that to encourage her to go....so now we've tried to tell her it's now for going for wees in the potty. We have also promised her various things that we know she likes like some particular toys when she first goes etc, I have also for ages now let her watch me use the toilet, I've even sat and weed on the potty to show her it's easy/not scary.Finally a couple of weekends ago I told her she needed some knicker time, she was very upset at first but then I explained to her that the nappies she has now are the biggest you can get (which is true) and soon once she grows out of them there won't be any other ones she can use. she listened and agreed to wear knickers, so for the last 2-3 weeks she has been wearing knickers for several hours a day without complaining, BUT still she will not sit on the potty. She'll talk about it, go near it, and say she will but then gets upset and says 'later, later'. Today she went about 4 hours without weeing in her knickers, and then suddenly she urgently asked for a nappy, as soon as she started pulling it up her legs the floodgates opened and she weed everywhere. We have tried so many times to explain that it's not scary, let's do it together, we have special toys and books that we've said only can be used on the potty etc, but it's like there's a bridge that we just can't cross. It's so frustrating, today I lost my patience a bit and tried to sit her on the special potty seat she has fitted in the toilet and she screamed and cried and I felt like I just undid all the good work.I just don't know what to do to get past this mental block she has. I also am worried that if we persist with the knickers and she continues to go several hours without weeing she's going to get a urinary infection. I shoudl add that she hasn't really spent a lot of time around other children, I now work full time but before that I did take her to the occaisional toddler group etc but I don't have any mummy friends I see a lot, nor has she been to childcare as she is looked after by me, my husband or my parents/in-laws so she doesn't really have any point of reference from a peer point of view. She starts pre-school in September and I think, personally, she will become interested in using the potty once she starts going there, as the toilet area is right next door to the pre-school room and we could always ask the teachers to take her in there when other kids are using the potty....and they are OK with her not being potty trained. the other important upcoming thing is that I am pregnant with my second child (due in November) which, even if we make some headway between now and then, she might regress once the new baby comes along. But to be honest I don't mind that so much, I can accept there could be some challenges there, but mainly I just want her to get over this mental block of sitting on the potty, it's driving me crazy and I just don't knw what to do for the best. She is very very stubborn and from past expereicne, pushing her into things never works.Anyway, any advice would be welcome! Thanks,Aimee