I'm so sorry this happened to you. It's so hard to have those feelings start to build up and really feel like you're going somewhere on a healthy and exciting path and then feel the carpet tugged from under you.

The most important thing is that you address that feeling of not wanting to open up, because it's so understandable but at the same time can be so detrimental to your approach to dating.

Presenting your true self to someone is an inherently vulnerable act, because if they reject you they are actually rejecting YOU and not some mask you're wearing. We all wear these masks pretty constantly, to varying degrees, but the one person you don't want to do that with is your partner.

Although it's scary to do at first, it actually makes the most sense. Think about this situation. If you hadn't been open about your feelings, you may never have found out about these feelings she has for this other person. Although it was a hard discovery to take, I bet you'd rather know that than not.

As others have said, it has nothing to do with you. If her perception of you did change because you were vulnerable, then that's actually a really good thing to know about early on! Do you want to be with someone who becomes less interested in you when you're vulnerable?

You approached this situation is a super mature and thoughtful way. To stop opening up because of this is the exact wrong lesson to take away, and I hate seeing great people like yourself become closed off because of an experience like this. It's hard to be vulnerable like that, but it makes you a real catch.

None of us know whether her perception of you actually changed or whether she just disclosed these feelings for another guy which she already had. But either way, it's information you're better off having. Dating is often dramatised as this euphoric, magical experience, but from what I've seen the strongest relationships are grounded from the beginning, with both parties being frank and open about both the good and the bad so that there are no surprises and so that they know that, if the other person is still interested, they are interested in you and not some rosy simulacrum of you.

It might help to remember that your feelings for her at this point are almost entirely a product of biochemistry and your relationship to yourself. You don't know her well enough for the feelings to stem from loss. It's a big deal though--some of the most intense heartache I've ever felt has been for short-lived romances. Maybe I'm weird, but with the long-lived ones at least you usually understand what happened. In some ways, when you barely know the person, it feels like a rounder rejection of you because when we know someone for longer we tend to be more aware of their flaws and so tend to understand the breakdown of the relationship as a product of your mutual dynamic, whereas with newer little things we don't know anything about the other person (who was probably not revealing their flaws) except the things we find attractive whereas we know ALL of our own flaws, making it feel like they rejected us because there's something wrong with us. On a pure biochemical level, it can produce some of the most intense feelings of loss and decimated self-esteem. Understanding this dynamic is unlikely to take the feelings away, but it can help you make sense of them. Understanding where these feelings truly come from allows you to honour and accept them without allowing them to twist and congeal into scars that impede your future romantic self.