Maybe the best way to deal with this would be to say what you have to say with the assumption that it will be ignored....
This way speaking up won't seem like such a big deal because you don't have to worry about all the attention being on you....
And if you aren't ignored it's even better isn't it and you can slowly say more and more things
I know how you feel about wanting to 'shout in their faces', in similar situations for me I feel as tho one day I'm going to flip and just let them have it! (*secretly cant wait till that day*)
But then you do have to start taking small steps now and again to achieve your ultimate goal, and if that means speaking a bit louder than you usually do or using more hand gestures or making more eye-contact with people then so be it!
Remember this quote:
"The reason people fail rather than succeed is because they trade in what they want at that moment for what they really want in life"