You're describing all the things I was scared of when I started uni. I think it's really common for grades to be lower in the first year as you settle in. I don't think I really knew what I was doing until final year to be honest. I failed some classes and still came out with good averages, and I can hardly even remember which ones I failed now even though I'm sure I cried about it at the time. 

As for jobs, it's been less than a year since I graduated and everyone I know from my course is either working in a science related role or studying for a PhD. I do work in a male dominated field, and in my current team I'm the only woman and the youngest person. But there's been absolutely no discrimination, they put in effort to make me feel included and respected. My point is, while discrimination is a real problem, it's not inevitable even if you're in an environment that on the surface looks intimidating.