I am sorry to hear that...I hope all is well with you...Something you may need to know, however, is that WAYYY before a woman utters the words "I don't want to see you anymore", she has already done so in her mind..I mean some women (although I am tempted to say most) have mentally broken off with their partners days, weeks, or even months before they actually come out to say it in reality...This issue of "I want to marry you" and 2 days later "I don't want to see you" is a hallmark of pre-meditated and pre-decided break up...She broke up with you way before she came out to say it...She was only seeing if there was anything that will bring her back to or hold her back in the relationship before she actually took that step...I know I may be wrong, but I speak from my familiarity with such breakups. I see the exact same pattern happen in those relationships.
It is not a question of whether you were a great boyfriend or not...Its that there are some things about herself or your person or the relationship that she is finding hard to swallow despite the other good stuff you have been doing to and for her...There are some aspects of the relationship that all this while, she could not take, but was waiting to see if she could somehow get used to...For her, the relationship may have been interchangeably Hot and Cold, hot being WONDERFUL and cold being filled with stuff that heavily distrubs her...When she left your house, she may have said to herself "Ok, i tried but no more"...
Not trying to be the genius in the class here but I just wanted to share from what I have noticed...Maybe next time you get a chance to talk to her, ask her what she did not like, or could not take in the relationship...Try not to point out "I thought you loved me, or I was a great boyfriend, or something like that"...Rather, try saying "I am sorry, I thought everything was going smoothly but I guess there must have been some things that I may have been blind to that hurt you or made you feel uncomfortable...Is there anything that has pushed you away? Is there anything that was overbearing for you?" When you ask those questions, you shift the issues on your self...But the trick in that is that if she opens up to these questions, you will slowly and gradually be able to see what went wrong, how best to fix them, and better still, you will be able to slowly discover if she also has any personal issues or problems that may have contributed to the break up...maybe trust issues or insecurities..When you finally get to the root of the issue, assure her that you are there to help her and if she is willing, you want to do what you can to help her, and maybe the relationship, if she is willing...That is, if you also want her back of course...