Truth be told, if you cannot tell your girlfriend openly that you think that she is less attractive to you know than she was before, it probably is not the best of relationships. Of course, timing, framing and phrasing matter, too. And don't expect her to be glad that you told her (at first). She'll be mad. But long-term you might reach your goal of her losing weight and create a positive externality regarding openness and trust in the relationship.
I wanna ask my girlfriend to lose her weight but can't
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First of all, if you're also overweight, you're going to have to "lose it together". You can't be eating 3500 cal/day of donuts while she's on 1500 eating salads. You can probably sneak an unhealthy meal or snack now and then, but you're going to have to fix your exercise and eating habits as well. If you eat like the average American male (poorly) you're no better than she is-- just luckier, for now, because your metabolism hasn't failed (yet) and because the male metabolism in general has more room for fsckery.
I think the answer is to speak very indirectly but with praise (or impressed-ness) every time you're out with friends and there's a female who has kept her body. Not, "She's so hot", because that's creepy and will upset your GF, but, "What do you think she does to keep her figure?" The mathematical truth, of course, is that weight gain or loss is calories out minus calories in, but the more you ask, "What do you think is her secret?", the more you will drill in the idea that (a) she can fix this, and (b) she should, without bringing attention to the fact that she is losing you (which she probably knows on some level, but nothing is gained by reminding her).
But when you do this, you have to dissociate from your own emotional and s3xual needs. Not, "You'd be so much hotter to me if X" but "It's impressive how she's 43 and doesn't have anything on her body that doesn't belong there." I know that "male gaze" is an annoying w0kist term, but disassociate with the male gaze and make the sort of comment a female friend would make.
I'm on the left in general but I hate how society has demonized male s3xuality. Unfortunately, it's something you have to deal with. If you directly confront her on this, you become the eidolon of male entitlement and now you are the enemy. Of course it is just a biological fact (not an artifact of patriarchy) that women, on average, can't eat as much as we do-- but one can't get away with saying this. So you have to be ultra-subtle.
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Promise her that if she loses weight you'll learn to speak English.
Learn how to use punctuations first.
If you find that sentence hard to read without punctuation then you really need to study up on the ESL.
Sure, you sound like a Chinese guy trying to pretend he's as good as a native English speaker. Whatever makes you sleep well at night, bruh!