Sorry you had such a lesson. However, look on the bright side. It is better that she says no now than discovering after marriage that she is not really interested - a lot cheaper too.
If you cannot get a good value for the diamond, consider hanging on to it for either the next one - or for a piece of jewelry that you might wear.
Quite honestly, I have decided up front not to buy the main center stone of a 3 stone ring until she says "yes" for just this reason. Yes we are talking, and yes we have agreed on what type of ring she wants, and all kinds of other things. But she has not said yes yet, and may never.
Now I intend to surprise her (I hope) next week when she visits with the sidestones for that 3 stone ring (1/4 carat super ideal cut, with medium blue fluoresence - that have taken a month to find - assuming they pass independent appraisial). However, if she never says yes then those can be set into earrings, cufflinks, and who knows what else (yes I have gambled some $ on this).
Wow, I''m so sorry to hear. But yeah, consider yourself lucky that she said no before you guys got married. So much more expensive when that happens.
My friend had something similar happen. Well, they actually decided to break up after he''d proposed (better I guess), and I think financially/emotionally/etc., he was much better off having had that happen before he got married.
He just saved his center stone. Got rid of the setting for real cheap. He recently got engaged again. Used teh same center stone, just different setting. If you can afford it, maybe hang on to your stone for a while if you really like it. As pretty much everyone here can attest, finding that stone you like takes a ton of time and a ton of energy.