We had similar issues for a while with our daughter and eventually got past them with a few methods, which among other things involved talking about others using the toilet, praise for when she used it and creating a little reward system.
Firstly, we use a child toilet seat that slots on top of any toilet so we can take it anywhere. Something like this:
If you don't have one, or would consider buying something new, let her pick one. You can get them in lots of styles, possibly in her favourite colour and even with characters from tv shows she likes, which might be something that makes her more comfortable with sitting on it.
We spent about a week with no nappies on except for bedtime, and there were a few accidents along the way. She pretty much seemed to have her BM first thing in the morning or when she got in bed at night, so she had her nappy on. Whether she was holding it or not, it wasn't clear. She would also sometimes ask to have a pull up nappy so she could do it, and like you we took it one victory at a time as she was using the toilet and potty (we had both options) to pee.
Talking about it
We used to talk about using the toilet. So we would say things like:
"mummy and daddy sit on the toilet when they need a pooh"
We'd always try to tell her when we were going, so it seemed normal, and sometimes joke about using the potty.
Me: mummy, where are you going?
Wife: I'm going for a pooh.
Me: are you going on the potty?
Wife: no, I'm too big for the potty, mummies and daddies sit on the toilet.
We found that she'd start copying us with these questions and jokes, so it was mildly amusing for her.
We'd also ask about other children at nursery, as the best encouragement can often come with seeing that children her age are doing it. So we'd ask her which of her friends were using the toilet at nursery. We also used to ask the nursery staff so we could say:
Aimee told us today that Elouise did a pooh on the toilet today.
Our daughter loves stickers, so we got a reward chart sticker book with some of her favourite characters. Every time she used the toilet, she got a sticker. We also bought some more special stickers as further incentive for when she wasn't using the toilet for BM.
We used to get her some form of treat at the end of the week, or when each section of the chart was complete, with a promise of going toy shopping when it was fully filled.
Show how proud you are
When she first started using the toilet, we showed her we were very proud of her with overly enthusiastic praise and plenty of cuddles and generally verbalizing her achievement.
We used to ask her to tell people why she was wearing a sticker. We would drop her off at nursery and ask her to tell her key worker what she did last night too. More often than not she was shy about it initially, so we would tell them for her, but they were great and full of enthusiasm, which helped. Seeing positive reactions to the news encouraged her, and eventually she was happy and proud enough to tell people herself.
We did try not to push it on to her and if she refused to used the toilet, we didn't make a big deal of it as we didn't want to associate her getting upset with using the toilet.
There were a few periods when she hadn't been for a few days and when the time came it seemed to hurt. We found that it seemed to coincide with her possibly not drinking enough fluid on those days, so we tried to ensure that she drank more and also increased her fruit intake.