Emtz, I was going to reply early on, but realized I needed to think about it first. Some of what I say will echo Fiznat (gotta give credit where it is due).
First and foremost, as Fiz said, confidence really does go a long ways. Unfortunately, too many EMT's have a dog eat dog attitude when dealing with others who are or appear to be weak, shy, new, young, and unknowing. Your post indicates to me you have some deeper personal issues you need to work out before you can fully work out your problem with your coworker. Maybe dealing with your coworker will help you to start dealing with those deeper personal issues.
Your coworker is probably satisfying her leader status by belittling you. I am not going to say to start a war against her actions but rather stand up for yourself. When you stand up for yourself others will realize they cannot 'get one over' on ya. Some of these things are just skills that come with time. I cannot say I have been there, done that, and received a T-shirt for it, but I do know it takes time to hone the skills.
It is nice to have someone you can work with and be friends with, but you have to remember that at work you should first be a professional. Standing up for yourself might not make you fit in with everyone, but that is not a reason to be at work. Do not let that 'work friendship' get in the way of your self respect.
Good luck to ya bro. Let us know how it works out for you. Also, want to say good job on your first post. Just word to the wise, get a feel for the site before you start going after people too much.
Just sayin'...
Matty