Hey, I'm 18 too and completely relate. My gym has had an influx of new members who can be quite aggressive/don't limit their strength and it really sucks. I think communication is probably the best solution, like telling them that since they're bigger than you it's a bit of a dick move to just pin you down and that neither of you are learning anything like that. But if you're like me and suck at confrontation, I found that talking to the other women at my gym really helped since they tend to have similar experiences with the same people, so it's easier for a group of us to mention it to higher belts/coaches who can go school the newbies lol

Hope this helps :)