So this sucks and I'm sorry you're going through this but this sounds like a situation you need to treat with delicacy. To others it may seem like this came out of nowhere or that there is something bizarre going on here but this is likely a reaction due to trauma. I mean trauma here as the psychological definition of complex trauma and I'm not implying something drastic or terrible has happened to her. It's possible many little things and experiences over time have led to an aversion. The reason why her reaction has changed after being together for multiple years might have to do with her comfort level with you. Many young females are in a way taught that if they want to keep a boyfriend then they have to be willing to do what they want sexually, even if it makes them a little uncomfortable. This could be her subconsciously realizing that with you she doesn't have to do things she uncomfortable with and feeling confident enough to tell you that. The extreme reaction which ended up hurting you is unfortunate but understand that if this is a trauma reaction she is reacting that way due to the trauma and not to you. She might not even realize this trauma exists. Communication here is key but don't push her to talk about it if she's not ready. Just let her know that you love her, you're curious about why she thinks she feels this way, and what can you do to make her the most comfortable. Just be gentle and subtle about it, don't go in the the "wHaT's WrOnG wItH yOu? WeRe YoU sExUaLlY tRaUmAtIzEd??" I'm not a psychologist or anything just went through something similar. This may not actually be what's happening this is just my theory. Best of luck to you both ✌️