<human>: Been together for nearly 4 years now. She is gaining weight constantly. Its now at a point where im becoming resentful and losing interest sexually. I feel terrible to feel this way; especially when shes so amazing and everything i could want for in a partner.
Ive mentioned it before and she gets defensive. Occasionally in arguements she will always throw it in my face that i apparently think shes a "fat fuck". 
How do i bring this up again? Its getting to the point where the relationship is strained yet i feel mentioning it will make hell break loose.

Ive suggested exercising together but shes very lazy. While im trying to go for a 20 minute run shes insistent on stopping every 30 seconds; literally barely working out at all.

Can you summarize the given text in a few sentences?
<bot>: girlfriend is gaining alot of weight. Mentioned it to her before. She refuses to lose it and always throws my previous concerns about it back in my face.