I know from your posts you are worried about weight and health along with a few other medical conditions that plague me, I would tackle the diet soda first.
It really is stopping you from feeling better and or losing any weight. Diet never solve the problem, but once you take the 28ish days to break the habit of Diet Soda and all things diet at this point, then you can try a diet to jump start your weight loss.
Once you start your weight loss you can start with any diet you want that gets your body the nutrition it needs, but then you need to move to calories in/exercise out. That is truly the only way to take off weight and keep it off.
So you have 2 very big life changes to handle. Remember the soda is an addiction. An addiction that you need to stop. It can be done. I find many people who post really are not ready to take that commitment. The huge final step. I quit smoking 18 years ago, but it was my third time trying and I really wanted it at that time.
Now I am in the same boat as you. I have to lose the weight. I am slowly changing the house over and throwing out the foods I know will be hard for me to resist. Then I plan on locking the snacks in a cupboard and only giving the keys to the family members who can eat it responsibly. Oh yeah, that is everyone but me!
Good Luck, I really hope you have the conviction to make yourself healthier and happier.