This fear is rooted from many different areas - some people fear speaking up for themselves because they always have had someone speak up for them, they fear to say the wrong things, they don't want the person to get mad or become confrontational with them, they don't care enough about the situation, or just they don't know what to say.
My advice to anyone who is feeling this way is to SIMPLY SPEAK UP! The saying speaks now, or forever hold your peace is so accurate. You loose so much peace when bottle things and the real vindication comes when you just let how you are feeling go. If you don't like the direction something is going - speak up! If you feel like someone is disrespecting you speak up! If you feel like people are attacking you speak up! When you speak up, things go better for you.
I had a huge relief when I released somethings that I have been feeling. It felt like I was at my breaking point and today I let people know how I felt and now I feel vindicated. For anyone bottling up fear and etc - just speak the hell up or forever hold your peace. I don't know about you, but I love having my peace.