Well good luck to you. It's not easy being assertive if that's not your natural tendency. At a minimum, I think you should respond to the "girl" comments with "I'm a grown woman thank you!" and put a dose of attitude in it.
With all due respect, Tightwad, this is really bad advice for a woman in this type of situation. Unfortunately, we're subject to double standards in the workforce as well. We're expected to be sweet and people walk all over us. But if we "put a dose of attitude" in standing up for ourselves, or tell someone to STFU, we're labled as bitches and we lack the ability to be team players. Either outcome means we don't get the respect we deserve from our coworkers and bosses.
I had a lot of the same challenges early in my career, Newb. It doesn't help that most of my male coworkers have daughters my age (ish) and that's how they relate to me.
It's great that you've seen some success with changing your wardrobe. Dressing in suits and presenting yourself with a more professional, "grown up" appearance will go a long way to help. Also, if there's a woman higher up in your organization whom you respect, you might want to consider asking her advice and developing a mentor / protoge relationship with her.
A book that helped me early in my career is Nice Girls Don't Get the Corner Office. It gave me lots of good, actionable tips to on how to present myself at work in a way where I wouldn't be viewed as "sweet" (gag) or a little girl, and how to assert myself in a manner that didn't get me labled as bitchy or difficult.
You're welcome to pm me if you want to discuss further.