I don't know how old you are Cat, but when I was younger, I was the same, people did take advantage of my quiet, polite demeanour, but so much crap has happened over the years at the hands of other people, where I've lost out, usually financially, that as I've become older, I've gone full circle.
My problem now is how to deal with things without being aggressive about it. I now have no problem telling people what I want or what I think, which can be a problem, as I'm sure I must appear confrontational at times.
You may need to take a self assertion course, so you can learn how to get your point across, directly and concisely. If you tend to dither when asking for something, it comes across as weakness and you will get nowhere.