I don't know much about writing, but studying maths maybe I can give you some ideas anyway. First: Just because someone likes science doesn't mean they just walk through it and understand everything because they are such a genius (in contrast to how "science people" are usually depicted in fiction). This stuff is hard. That is why professors in STEM degrees often start with a motivational speech. And you probably can use this kind of struggling to make your introduction more interesting. I mean imposter syndrome is really common in that field I think.
I am kind of writing this because of the answer with:
I'd say make the science part incredibly fun in the first third. Have some lab accidents, some funky experiments, liquid nitrogen on hand, helium, goofy stuff she plays with even though she shouldn't.
If you want people to get into STEM, if you depict STEM like that, they will feel like they are bad at it, because their experience will be different. Because this is not the experience you have when you do STEM. Even if you love it.
A few anecdotes:
My Analysis 1 professor started, by drawing a "yellow maths heart" with a marker pen on a piece of paper, which was projected on the screen, and said: some people will love maths some of the time, and sometimes they will hate it. You will never love it all the time. He then said that we could now mark him well in "media usage" in the teaching evaluation, because he used a projector. (Maths lectures are blackboards only basically)
He also distributed a package of jelly pigs at some point and said: don't eat it yet, keep it until your first year is over. Whenever you get stuck, punch it, remind yourself that you can do it.
You don't even understand the lecturers in the first semester, if you start to be able to ask questions in second semester (because you understand enough, to know what you don't know), you are basically already in the top 3%. Sometimes you will work on an exercise for hours, give up, try the next day, give up again because you have no clue, start googling, and then find it on wikipedia and realise that it could be proven in 2 f**ing lines. And on other days you will solve a "hard" exercise in 10 minutes.
And when you think back about the first semester you always think, man that stuff was easy! Because you got so used to that way of thinking. It is similar to how arabic numerals make calculations easier compared to roman numerals, because you can use the decimal places to divide and conquer the calculation. Similarly other definitions and concepts will enable you to do things which seem more complicated before. But for that you first need to understand those abstract definitions. And get used to them.
But once you understand those concepts, it become really easy to use them and you can't remember how hard it was in the beginning.
It is like learning a language by only listening to it. Because you can't explain the language maths in a language that is not maths. Once you understand the language it seems easy. And people around you treat you like a genius or something, even though you are just doing some easy things. That is where the imposter syndrome comes in.
And in lectures it is always so easy to fall into the trap of: I don't want to ask this, maybe it is a stupid question and I just don't understand it because I am stupid. And since everyone is afraid of the same no one asks and everyone keeps quiet. While the lecturer has no idea that he just left the entire hall behind. There were multiple times I almost didn't dare to ask a question, made myself do it and then the same question was asked by someone in the next lecture because they apparently didn't even get the question in the previous lecture.
It is probably hard to depict this. But if you want a realistic story, then you will be better off if you don't make it out to be a piece of cake. The fun you get out of STEM is like the fun you can get out of sports. It is gruesome most of the time, but when you manage to do something it is all the more rewarding. And some people like that ;-) And aren't stories supposed to be about struggles?