U do the diet exactly what u expect her to do. When ur appearance more attractive then before, she ends up 2 situations, she cannot feel anything still doing her own business, u two ald in separate world, so naturally breakup, second is she feel danger and shame, join ur routine, u hit ur target.

However there is also third situation, u end up to have more attractive appearance and u decide it's time to change partner, u initiate the breakup first, haha. Means u dont play enough, u dont wanna settle down actually, u thought u love ur gf but is actually not. u just want a slim "partner" that can give u face. Personally think u are this type, most of the men is this type.

Point is u must do the diet first and u have successfully improved urself. This is the key.

U know what, u can just break up, let her go.