Dating too fast too soon
Am I attractive enough for him (generally, not just physically)?
Does he truly believe I'm worth pursuing a relationship with?
Q: I just got dumped by my girlfriend a couple weeks ago.
It was a short relationship (3 months) which started out slow but gradually ramped up in intensity as we started spending more time together.
Break-ups, as you know, are not easy, especially when the relationship seemed to be going so well.
You are asking essentially two questions here: 1) why she would have a change of heart so abruptly, and 2) why such a short relationship could hurt like a much longer term relationship. Given what you describe about your ex’s behavior, it is possible that she terminated the relationship because of having an avoidant attachment style, meaning that she is fearful about entering and becoming too close to others.
This has helped to foster a healthy attitude to resolving conflicts (which we do remarkably well) and admittedly is also a result of a combination of some very important, complementary character traits.
Too intense too soon stunts the development of a good, strong, healthy relationship - mainly, I think, because high levels of physical intimacy and all the hormones that come with it can be a distraction (the strong biological basis referred to here by the author) to the nature of the relationship itself as it would work in the long-term, given the combination of the two worlds about to come together to create something beautiful - building a solid foundation, friendship, trust, common interests and values, getting to know each other on a deeper level and so on - (although I'm wary of making causal claims) and can potentially be very confusing when that calms down and the reality starts to slowly shine through the cracks.
You may also want to try out being sort of a “mystery man.” Many women prefer dating partners who are not very obvious about their feelings at the start of the relationship because it violates their norms about what it means to be “manly,” In the long-term, warmth and responsiveness are good, but keeping it low-key at the start cannot hurt. If you are talking to her about future plans together at month 3, then you, as the man in the relationship are moving way too fast and it will cause her interest in you to decline. Good points, Jack-In fact, the "mystery man" approach you advocate is exactly what the scientific research I cited at the end of my answer suggests doing.
If it makes you feel any better, she may feel guilty or bad about rejecting you, as rejecters oftentimes feel as bad about ending things as the person who was dumped. Harman's research examines relationship behaviors that put people at-risk for physical and psychological health problems, such as how feelings and beliefs about risk (e.g., sexual risk taking) can be biased when in a relationship. I agree that this may be the most likely reason she backed off.
I was totally blindsided and heartbroken by her actions.
How can someone seem so into you in one instant and decide they don't want it in the next?