My girlfriend [20f] of 18 months has gained a lot of weight and I [26m] find it very unattractive. How to handle?

My girlfriend and I have been dating for 18 months and everything about our relationship is great...she's still as very kind as she's always been; we still have a lot of things to talk about; I still love spending time with her; I still think she's the most attractive woman I've ever seen; etc. About 3 months ago, she started gaining a little weight, and after a few pounds, I started to get a little concerned. We had a gentle conversation about it and she agreed to start eating healthier and working out more. Well, 3 months later and she's gained a substantial amount of weight. She's gone from being extremely in shape to having a large stomach, another gut under that, and fat all over her body. I've tried to ignore it, but the more she gains, the more I just don't find her attractive, which makes me not want to have sex with her.<br><br>We've had sex twice in the last 3 months. I've been masturbating extensively because I don't want to have sex with her, and I need to satisfy myself somehow. I've told her she needs to start losing weight as soon as possible. I've also tried to be more supportive in the weight loss process, such as making sure we're both on the same healthy diet and occasionally going for runs with her to get in shape (I don't need to get in shape, it's for moral support).<br><br>She's still gaining weight at a rapid pace. I'm stuck in a difficult situation now because she's not losing weight and I don't think I can stay in a relationship with someone I don't find attractive.<br><br>What are my options here? Can I approach her about this and demand she loses weight or else? I don't want to sound insensitive or selfish. I feel like just letting it go and staying in the relationship isn't an option.<br><br>After posting, I want to clarify a few things:<br><br>1. My attraction to her is 90% physical and 10% personality. I can't help but be superficial. Being attracted to the physical body is a crucial component of any romantic relationship.<br><br>2. I know it's worse that I've been masturbating. I feel guilty. She thinks I've lost interest in sex because of stress from work. I'll probably never be able to admit this to her. I feel like an ass. I do want to have sex with her, but only if she starts losing weight.<br><br>3. I would love to be supportive, but I've made my best effort already. She knows I've been sticking to the healthy diet with her because I want us to be healthy together and I want her to lose weight super badly. Once or twice a week she orders delivery and gains another pound. She also says she's going to the gym a few times a week, but is clearly not because she's getting fatter.<br><br>EDIT: She went from being a size 2-4 to a size 10. So it's quite a large amount of weight. I know this is shallow but I just can't help but be turned off.<br><br>EDIT2: I understand people are upset that it's not a normal case of "girl gains 5 pounds during quarantine." This is a girl who went from being a size 2 to a size 10.