I have a similar problem. In general I always find it hard to speak up in a lecture or seminar, especially since my course calls for subjective answers (english and creative writing). I can count the number of times I've willingly contributed to the whole class without prompting on one hand during almost three years, and when I did it was all over the place really. Usually because I think my opinions on literature and concepts aren't obviously going to be as good as people who get it straight away and supplemented it with tons of reading beforehand, I just don't really feel it's worth me saying anything.
I felt really stupid yesterday, as we were doing group work and I haven't finished reading the text so wasn't very productive, in a word. I guess when I do do the reading I can contribute fine within a group, it's just relaying that information coherently that's the trouble.
Sorry if I'm hi jacking, but my advice entails something I'm trying to do too. Just prepare yourself more for lectures and seminars, and don't be afraid to ask anyone questions if you get stuck. It is third year after all. It'll prob give you more confidence in the long run!