I think what is a 'deal breaker' in love/relationships is very different to all of us.

Some people find physical appearance a more important trait than others, you seem from your post to be one of those people. There is no magic formula I know of for changing that within yourself (because we can only change ourselves, we cannot change others).

I think you have a couple of options, you can either compromise and accept your gf as she is which is not your ideal body shape and put that aside in favour of her other amazing traits, or you move on and look for a partner who is a better physical fit for your preference. I don't see trying to change her as an option for the reasons very well expressed by the previous posters.

I would like to add (as someone who is very likely a good bit older than you) that physical appearance changes drastically as we age - in a long term/life partnership it's wise to be aware of that. What keeps older married couples together and happy is not usually the shape of their butts but compatibility and a life time of team work.