Help! I have an issue with my dress fitting. What do I do?

Hi there, My wedding dress is a strapless sweetheart neckline design that fits like a glove but I am quite concerned about the gapping at top bust area, especially when I team it up with a strapless push up bra. I am thin on top and god has given me very little in terms of cleavage. I have tried wearing the dress without a bra and the gapping is less as I'm able to tie the corset back tighter. However I look flat with no cleavage. What to do?