A question for all of you moms...

I'm 17, and still living here at home. Everyone in my family is obese... my mom, aunt, grandma, ect, and everything from heart disease to diabetes to thyroid problems runs rampant in my blood line. One of my little brothers is well on the way to becoming obese, and the other, even though he's skinny, has terrible eating habits.
Here's the thing:
I've decided that this is the year that I'm going to get into shape. I'm getting married in about a year, so I thought it would be nice to start off my marriage without the extra stress of trying to lose weight, or the stresses of the medical problems that come with being overweight. My mom is always saying that this week is going to be the week for her, or next week... the diet always starts tomorrow. I'm not here to down her, because I think everyone knows that feeling. However, I don't have her full support now that I'm trying to lose weight.
Earlier this week, she agreed to let me cook and told me the general things that sounded good to her for dinner. Tonight we went shopping, and I had made a list up that had five dinner recipes on it, plus general around the house things. She started freaking out, telling me that we don't have the money to buy "10 items per recipe", and that if that was the way I was going to cook healthy, that she wasn't going to let me do it at all. In reality, each recipe had 5, maybe 6, items at the most, and a lot of the ingredients were little things like cans of beans or frozen vegetables.
The grocery bill freaks her out, because this is how she generally shops: she loads up on $30 or $40 worth of frozen meals to take to work, buys a few odds and ends like cereal and pancake mix, and then when we don't have anything to make for dinner, we go out to eat two or three times a week -- not including weekends, when we might go out two times in the same day. When we go out to eat, we go to decent restaurants... the normal bill would be $50 for the four of us. She refuses to see that I could have cooked at home for a fifth of the price, and made it fifty times healthier.
The problem isn't money, so I don't know why she claims that... she would actually save a decent amount by buying fresh ingredients instead of going out to eat. We can go on thousand-dollar shopping-spree weekends in Vegas, so it's not like it's not there. The problem isn't the cooking, because even though she hates cooking, I LOVE LOVE LOVE to cook, and since I'm home all day, I'm more than willing to have it done by the time everyone gets home. And the problem isn't that I cook badly, because everyone always raves about my cooking.
Now, moms, maybe you can clue me in this: Why is she so hesitant to just buy good, quality ingredients and eat home-cooked food? Why is she hesitant to eat better and feel better, possibly getting to get off of her high blood pressure meds? How can I try to ask her to keep buying fresh ingredients and to let me cook at home?
In case anyone mentions it, I would get a job to pay for my own good food, but I can't... I don't have any means of transportation, and no one is hiring, anyway...
Thanks!~