Well, being about your height, I would steer clear of anything with a floral design. In my opinion floral designs only suit very young women of our height, although the look nice on taller women of any age.
And knitted dresses make me feel that my tummy is sticking out!
Helpful, aren't I?
I would start by going round the charity shops in the area and trying on anything and everything I could find that looked remotely suitable - in other words that appealed to me.
I would also dig out a pair of high heels and brush up on walking in them, as frankly I feel so much more elegant in high heels whether with a dress, a skirt or trousers.
If when you say bottom heavy, you literally mean you feel your bottom is on the big size a pair of high heels is the answer, as you walk differently in them.
Mma Makutsi in The Ladies' Dective agency series mentions " the delicate matter of what to do with your bottom when walking. Some people just leave it to follow them, others use it" or words to that effect.
Use it - don't attempt to conceal anything you regard as a fault.
If you really feel your entire bottom half is a size larger than your top wear a padded bra for the ocaision.
Actually, a padded bra will balance your large feeling bottom too. (It cannot possibly as large as you feel, if you can get it and the rest of you into size 12)!
Find something you like and feel comfortable in and enjoy the wedding.