my situation is the opposite

my wife exercises regularly and maintains a strict discipline diet.

she asked me to do the same but i cannot cope with it.

To go on a diet, it requires very strict discipline.

If your gf is not someone with self-discipline, you wait long long la 

When I first met my gf she was slim then she keep on gaining weight. She has unhealthy eating habits and dun exercise. Anyway to ask her to take care of her weight any argument to use to change her behavior. I tried but didn't work asking her to be healthy.

You need to lead by example. If you cannot lead by example, it will not happen.

You need to start adopting a healthier lifestyle and pick the places you go to eat with more healthier options.

If you opted for the healthier food, she will indirectly follows. If you opted for more frequent of walk at the park or housing area, she will follows.

The most importance turning point to push it to self sustain momentum = she must be able to see the changes happening to her body, her waist line reduces, she can wear more attractive clothing with more confident.

That is usually 6~9 months gestation period. If you cannot endure pass this period, usually the subject will be demotivated & loss the momentum.