Why other people do no respect me?? :((?

Hi, I'm a good looking girl, but I'm quite shy and not very self confident....I come from Spain and I work in London. At work I cant make myself respected with my colleagues...I feel that maybe I'm too "nice"saying always please and thanku....I'm shy and I dont speak my mind because I want to be polite and diplomatic..the thing is..when Im attacked I just dont react....maybe I cry later alone in my room ,but I dont react in front of other people and when I do I cause a fight!!!I dont know why but other people dont respect me and this makes me feel depressed...what can i do to make people respect me? please help :(