Is this dress too formal for my wedding?

I am having some anxiety over whether or not to buy another dress. We are planning to get married in October 2014 in a city hall ceremony. When I was doing my research I read everywhere that you cannot have that big wedding dress you’ve dreamed of all your life. I accepted this and began looking for alternatives. I recently purchased a summer dress online that dropped above my knees.(picture1,2)

After trying it on over and over again over the past month I’ve begun to feel that it is just too short and feel defeated because I can’t imagine finding a beautiful white dress that makes me feel like a bride. I was browsing on eBay today and fell upon a formal long dress.(picture3)

It is the perfect size and perfect price but that little person in my head is telling me that I will make a fool of myself if I show up in a long white dress… but I also feel that all of my wedding photos (going to a studio afterwards) aren’t going to have a wedding feel to it with a shorter dress.

It would help to mention that I also suffer from a social anxiety disorder so it is killing me to try and make this decision as there is the potential I am going to be overdressed for the ceremony. I am also thinking perhaps it would be better if I could bring it to a seamstress and have it hemmed to be slightly between my knees and ankles.

Any feedback on this matter would be very much appreciated!