why is this happening?

Whenever I mention the relationship issues to anyone (and I haven't mentioned to too many yet), there is a question immediately asked or implied, and that is something along the lines: "You two seemed to be doing so well, what happened to cause this rift?"

Unfortunately, I don't have an answer. The immediate assumption is that she wants to get married, and although I've considered that, I don't believe it fits the situation - especially since I was planning to propose, and whenever we talked about marriage, me revealing an inclination towards getting married only seemed to make her more disconnected as opposed to getting closer.

From there, my mind jumps to a fear that I'm not worthy of her. However, that really doesn't make sense either. I've always treated her extremely well, and just as I've been happy to be with her, she should (rationally) be happy to be with me. I think we have to be a relatively good match in order to have 3.5 happy and harmonious years together.

I finally came up with the three best reasons that do make more sense to me

• stress of my Dad living with us, new management position / weddings / vacation travel
• Even though he's moving out this week, having a parent live with a couple for 8 months is definitely a strain on any relationship
  
• She recently took a promotion to an extremely stressful manager position, we've been planning for a couple friends weddings, and also squeezing in vacation travel to visit my family in Chicago and Michigan.
  
• Under this type of stress it's only natural to start thinking about your life, longer term goals, etc. Not only that, but she has been meeting new, interesting people in new position. Definitely a time of transition for her.
• I didn't fully realize the impact of this until the last month, but even since the beginning of the relationship, she has been keeping certain walls between us. I assume to keep herself protected - but that also meant that she wasn't as connected to me as I thought. Whenever the topic came up, I avoided it instead of working with her to break down these walls
• At the beginning of the relationship, I was a bit of a challenge to her. To some extent a challenge in a partner is good - it keeps interest up and allows growth. After she realized I wasn't a challenge at all anymore, and she wasn't feeling more connected than before - it was only natural to look for the next challenge

They are not as easy to articulate as "I think she wants a ring", but at least those three reasons give me something to work with / stand on. For right now, I'll focus on trying to break down the walls - maybe that can help us get closer again.