There is a weigh to do this, but there are a few things that you need to get straight about this situation to approach it sensitively and with love and compassion:

1) You need to be aware that this is about your needs. You are losing attraction to her because you find overweight women unattractive. Of course you are not alone in that sentiment, but this is about what you need from her in the relationship, not what she should do. You don’t get to tell her what she should or shouldn’t do. She is an adult, not your child, not your student, not someone in need of your unsolicited advice.

2) You need to understand that she is currently ok with her weight. I’m sure she is not happy with it, I’m sure she understands the health consequences, I’m sure she imagines on some level you’re not thrilled about it. But she has decided that whatever benefit she would gain from losing it is not worth the pain of whatever dieting and exercise would be involved in losing it. What you are doing by telling her this is that the costs of remaining overweight (your losing attraction to her) are perhaps greater than she realized, and it is now her decision whether it is worth her doing something about it. It is not something she needs to do. She gets to decide whether this is worth it to her and you get to decide how to respond to her decision. This is how to conceive of this in a way that respects both of you.

3) You need to eliminate all anger involved. You cannot be angry at her for gaining the weight. It is not something she has done to you — it is something she feels she has needed to deal with the stresses of life, her self esteem, etc. You also cannot be angry at yourself for having lost attraction to her. This entire enterprise needs to be motivated by compassion for yourself (that you have the attractions you have, and you need to respect them) and your girlfriend (that she is doing the best she can and her weight is not some kind of personal affront to you but rather a result of how she feels she needs to live her life to cope with it). The fact that she is so sensitive is an indication already that she likely gets easily stressed, hurt, intimidated, etc and therefore needs to eat to comfort herself or avoid the stressful thoughts. This means you need compassion not only for her needs but also what will likely be her sensitive reaction to you — her sensitivity is a defense, this is just how she is. You cannot get upset with her for being upset with you. You cannot say things like ‘Any guy would feel this way!’ or ‘Was I just supposed to pretend I was attracted to you!?’ etc. You are going to need to expect she will be hurt, you will need to give her space to process this, you will need to support her through the pain, you will need to reassure her that the reason you are telling her this is because you care about her and the relationship and you are doing this to allow the relationship to continue to persist.

4) Once you have gotten through these initial reactions, you will need to help her lose the weight. She will be doing this because she cares about your needs, and if you truly care about her and the relationship and getting your your needs met, you will need to find a way to lower the costs for her of exercising and dieting, since the current costs seem to her to her in terms of time, energy, pain, and potentially actual cost. This means cooking healthy meals, doing meal planning, exercising together. It also means setting very, very reasonable goals and those goals should start with slowly, slowly changing eating and dieting habits. You are going to need to have a world of patience here. The moment you start applying pressure to her to lose weight faster or make changes faster is the part where you’ve track of compassion and that is the moment of failure.

Good luck.