She doenst know if im the "one"

[T-ROD]

Im sorry for how long this is but I think in order to answer my question you need to know some background.

 

Here goes. My girlfriend and I dated unexclusivly (mostly on her part) for 2.5 years. She had just gotten out of a relationship that hurt her really bad and had some serious trust issues and then she was a senior in college and didnt really want a relationship. I stuck with her during all of that and the last six months we have been officailly together. We have had our ups and downs like anyone else but everything else is perfect. I love her with all my heart and she loves me but recently shes been thinking about marriage a lot (i think because a couple of her friends are getting married soon) anyways neither one of us ready for marriage at this point in our lives shes in vet school and im in grad school (no time) but i know when the time comes for us to take the next step in our relationship that i could and would marry her in an instant without question, but just last night she said she had been thinking about the marriage thing for along time and part of the time she cant see herself marrying anyone else and the thought of it makes her the happiest person in the world, but other times she said shes not for sure if im the "one". We both believe dating seriously someone you cant see yourself marrying is a waste of time and now i feel our relationship is in jeopardy. She also dropped another bomb on me at the same time. She said she has felt very guilty for me being commited during those 2.5 years when she was not and now she doesnt know if she has stayed with me because of her guilt. She has decided to let the guilt go and see if her feelings are different with me without the guilt. What does this mean and what should i do. I love her with all my heart and dont want to lose her. Thank you and sorry it was so long.