As someone who just got out of a long term relationship with a girl who had the same issue, my advice is to get a new gf too. Foreplay and all that stuff is nice, necessary even, and fun, but either shes into big dicks or shes not, (or indifferent), but either way-THAT WILL NOT CHANGE. Of corse you can try to change your approach, but Im sorry, if shes is not turned on by the sight of your wang, and its going to get a lot harder when you meet the many girls that are. Avoid the frustration now.