How do I approach my girlfriend about losing weight?

When I met my girlfriend 2 years ago, she was 5'6, 132 lbs. After dating for a while, she got a job as a nurse. Her schedule is so hectic now, working and going to school part time. She has no time to eat cooked and eat healthy meals so now she eats out of the vending machines and fast food restuarants. My girlfriend started gaining so much weight now, 28 lbs to be exact. I'm in love with her and I'm not going to break up with her cause she gained a couple of pounds. But I prefer her slim. I tried to get her to go to the gym with me but she always wants to spend her free time lying in bed. She does work hard. I brought us some bikes and she started riding for a month but again she says she's too tired to keep it up and she stopped. In my line of duty, I have to stay in excellent health. I wish she would too. How can I tell her to lose some weight without making her think I don't love her anymore?

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