walk all over me..

Over the years, I have noticed a general trend of people bombarding me with information that I am already aware of. Perhaps I just have some sort of personal issue, as if I take it as a personal attack when people try to tell me very obvious things. Back in high school I had a friend who always dumbed down things for me all of the time. I always wondered if that was just her personality and never paid any attention to whether or not she did it to other people, too.

Then, I had a roommate a couple of years ago who literally thought she knew exactly how everything works. She once explained to me that I should try shoes on before I buy, wiggle my toes around and then if I felt like they fit me well, I could buy them. The most insulting time was when I was in Target buying a printer cartridge and I had forgotten to look at the printer type before I left. I called to ask her to find out what printer I had and she explained to me that printers have "special numbers" that correspond to the printer cartridge so that you know which cartridge is compatible to the computer. Obviously I knew about these "special numbers", I just had forgotten to write it down.

Now, there is this kid at work who yells over any conversation that I am having with another person, to say pretty much the exact same thing I am saying, just louder and in different words/order. This kid tries to intercept every phone call they're present for and snatches things out of my hand to take over whatever task it is that I'm working on. The problem is that if i try to tell this person that they are stating the obvious or are annoyingly overbearing, they will take it way to personally and never help me again.
I wish I had a way of watching myself to see my body language, facial expressions and demeanor. I would like to figure out how I can stop letting these kinds of people walk all over me all of the time. I know that I have a more passive personality and that I ask questions, but why does that combination lead to people negating the idea that I have any knowledge at all?
I despise people who are masters of the obvious. They are the ones who not only tell you to bring an umbrella, but explain that if you don't have an umbrella, you'll get wet in the rain. I can't stand that. There are times when I need help, and I appreciate it, but there are also things that I am capable of doing and don't need my hand held to accomplish every little thing I do.