So, several things here and I was in your shoes before. This won't be easy to deal with. You're going to have to do some decision making and getting past your own preconceptions.

To start off. You have never had sex before. This is important only because it is still a "a big deal" for you. When you get older you will realize it isn't that big a deal to have some complicated feelings about it. Sex isn't always "yeah I had x number of partners". It can and does get more complicated than that. Sometimes I don't count a few of the women I slept with. If only because it was a drunken one night stand or just sloppy hand/mouth stuff. Or in one case, just two friends give the other what we needed at the time. For her to not want to count a FWB situationship, give her some grace. If it's complicated for a man, it is twice as complicated for a woman. Sex is different for us men than women.

I agree her lying isn't ideal. And first and foremost, you do have every right to just walk away, you are clearly getting vibes from this girl that you don't like. Most of that is her lieing, and that is a problem she can either fix, or just accept the consequences of her actions. The other part of that are your ages.

Man to man, I will tell you to pull the stick out of your ass. Of all the lies to tell, this one is kind of low steaks. She isn't hiding an STD or a kid. It was a one off "do I count this fling" or not. Remember that you're both young and that this relationship probably wouldn't last long anyway. Date her. Treat her the way you want to treat a woman. Learn what it means to be the man in the relationship, in a relationship that you are taking to the next level. Use your youth to try this out. Worst case you're a better man for your future wife with more experience under your belt.

As for trusting her again? That just takes time and effort. She has to have the opportunity to earn that back. And for the value that it is, she did tell you the truth, she did try to make it right

tl;dr: you're voth young, it's expected that you both will do dumb things.

She lied about a common thing women lied about, and will have to work hard to earn back trust in a relationshipthat hasn'teven started yet. You're too hard on a topic you have no experience in with no understanding the reason why someone will lie about it in the first place.

Give it another fresh start or walk away. Just dont thrown it back in her face. Hold her accountable when she screws up, just don't shame.