Well, there are a few options you have for eating better at Disney.
First off, if you are not getting the
DDP you have the option of ordering kid's meals for yourself at some of the quick service places. They come with a choice of 2 sides (carrot sticks, grapes, applesauce usually) and the portion sizes are just right for dieters. The problem with this is that the options for the main course are pretty limited (burgers, chicken nuggets, pb&j etc). A few places have different options though, you just have to do a bit of research on the menu listings on AllEars.
If that doesn't appeal, you can try sharing counter service meals with someone else. I find that DH and I can easily share an adult entree at most of the quick service places and I am NEVER hungry for having eaten only half. If he gets hungry later he gets a snack to tide him over to the next meal.
Also, many of the counter service places offer apple slices as a substitution for fries so that saves some calories and fat right there, even if you only do that.
It is possible to find healthier options (such as wraps and salads) but you sometimes have to hunt for them. Again, check out AllEars for update menus.
In terms of table service meals, I think buffets are a good idea because you can load up your plate with fruit and veggies and then take small amounts of carb and protein offerings. Of course, the "all you care to eat" option sometimes makes it difficult for people to resist going up for seconds and thirds (which defeats the purpose of going there in the first place) so only you can decide if this is a good idea for you.
And lastly, in the non buffet table service places, if you skip dessert most of the time (or just steal a bite or two from your DH like I do!!) and only eat half of your entree, you should do okay! You can also request sauces and things on the side to cut down on some of the fat and calories during your table service meals.
Make sure you drink plenty of water and you should be golden!!
HTH!!