Speaking as someone who had a multi-year relationship with no sex involved, I understand how frustrated you must be, OP. But being blunt with her seems like...really not the way to go. Sitting down and talking openly will likely help, but if she acts like she did in your example it may just cause her to panic and shut down or push you away, so I'd be careful on that front.
Ultimately...well, from your story I'm seeing a lot of different things. Some of these things are slight generalizations and I apologize for making them but as I've worked with more and more people around your age I find them to be true probably 75% of the time, so I'm not saying you guys ARE in these situations, I'm simply throwing them in as possibilities. Maybe even strong possibilities.
Being 18, 19 years old there's a lot of changes in life. For the first time you're an adult. You've got a lot of stuff that suddenly you're on your own for. Bills, Education, there's just generally a greater level of agency for you at that time. All of a sudden you're free after 17 years of having people making decisions for you. Some people embrace that and love it to bits. Some people (like myself), find it terrifying because suddenly the full weight of the world and even the mundane things of our own lives are on us and we may feel like we're not prepared for that. Add College onto that and I'm sure many people are very confused. Not only do you have the full spectrum of life suddenly in your face but now even the one constant that you had before, school, has changed. Depending on where you go, teachers don't care if you come to class or not, you make your own decisions for when and where to go to class, you make your own decisions on what to take, how to do things, much more of schooling is put on you now. Add on to that the traditional stereotypes of college and things get even MORE interesting/terrifying.
I don't wanna say that you guys are young, but...you're young. It sounds like she's unsure of herself, and as a relatively freshly-minted adult she's likely adjusting to a lot of things. Not everyone hits 18 and goes into the Party Phase (lord knows I didn't) that seems to last until about 24, 25.
Talking with her is probably a good start, as is observation: take stock of what you know about her and her behavior. The way she acts about other things may grant you clues into why she's acting this way about sex, as may anything she might have told you before such as religious affiliation, home or family life, personal values, etc. Obviously I'm not saying stalk her, but just pay some attention when you're hanging out together and examine the past times you've spent together and see if patterns come up. insecurity, trying to figure out her sexuality, anything of course is possible, but you may find some breadcrumbs to the reason simply by being observant. Not saying you should try to have her all figured out since the only person who can ever really know you 100% is you, but if you have a few pieces of insight, they might help you and her if you can get into an open, honest conversation.
I'd think the goal would be to, hopefully through trust and non-sexual intimacy/comfort with eachother, to be able to broach the topic in a way and time and place that allows her to be comfortable with talking about it. The goal is also to determine if that can ever happen BEFORE you try it.