READ BEFORE You judge. I just want her to lose weight because she wants to. I plan to make her want to lose weight for herself by making her self esteem as low as it should be. She's not really fat but she's not model skinny either.
I read it and I'm still judging. Model skinny is not healthy at all, and if she's "not really fat" then she might be at her healthy weight. You say that she wants to lose weight, yet you say that you want "to make her want to lose weight". That doesn't make sense, it sounds like you are the one who wants her to lose weight. To do that to a girl is the worst thing possible, to not make her feel good about herself, and you honestly do not deserve to be anyone's boyfriend. You should love her for who she is, not her weight.
The only weight she needs to lose is you. A good boyfriend would tell her she is perfect just the way she is. If she wants to lose weight for herself then support her by eating healthier and working out together. You never bring a person down, that is emotional abuse. You should be the one who brings her confidence up not down.
Making someone feel like utter sh*t about themselves isn't going to motivate them to lose weight. In fact, it's going to do the complete opposite. Also, the fact that you want to "destroy her self esteem" is a scum bag thing to do, she's your girlfriend for Christ sakes, you should be there to support her, not bring her down.