My gf is getting kind of overeater! Now she does love overeating.How can I stop her?

In my opinion if your girlfriend likes to eat you should let her, though however, if you are worried about her gaining weight you should suggest to her that she should join a gym or take walks (or do any other exercise e.g. riding a bike, running, swimming) half an hour after eating as this will help her lose the excess calories and burn the fat.

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(You should still love/like her even if she adds a couple of pounds though :) )
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Hope this helps,

S.J