First of all...you do not buy a $12,000 anything without insuring it. When I purchased my last car I wouldn't even drive it off the dealer's lot until my insurance was in effect. Why on earth would you take anything that expensive out of the store you purchased it from before it is insured?
Second, if she really loved you, wouldn't she just be thrilled that you had such a romantic idea...even if it failed? Either he has let her down over and over in the relationship so many times that her reaction is understandable in the total context of their relationship...OR...she's more interested in the ring than marrying the love of her life. Just my two cents.