A really good eating tip is....only make enough, or take enough serving....to be the meal/snack--and when the meal/snack is gone, you're done.
I'm not advocating snacking, but everyone has read about portioning out something into individual portions from a larger bag (pretzels lets say), and there is a real validity to that. If that is something a person can't do, then individual servings are the answer. Applying that mindset to your meals, you don't make leftovers. Leftovers can easily become second (and third?) servings, so if there are none, the meal ends, you're done eating.
In that same vein, when the meal is over...sit for a bit and really let your stomach signal to your brain that it is full. Drink some water afterwards. Heck, drink some water before.
A good sized salad before dinner will help you to be more satisfied by the main course. Even an appetizer of something like stuffed mushrooms or soup can fill you up, and keep that satisfied momentum going.
You can make substitutions for full fat foods easily. Step down from cream to milk, from whole milk to 2%, from 2% to skim. Light cheeses from full fat. Greek yogurt for sour cream. Light whole grain breads. You don't necessarily have to go the sugar free route--you can go the light route.
As to sometimes not wanting to eat, and others not being able to stop yourself, you need to make a plan. Failure to plan is planning to fail. Perhaps it isn't a matter of winging each meal, or taking a "wait and see" am I hungry or eat when I'm hungry attitude. Lets pretend you were enrolled in a health spa program for weight loss for 6 weeks. During that time, the food would be prepared for you, most likely 3 meals a day and 2 snacks, based on foods you like--but healthfully prepared--and within a calorie range optimized for weight loss for a person currently weight 340 pounds. Someone with less to lose at the same spa with you would most definitely be envious of your meals, cuz you would get more to eat. It is about dieting smart, not hard. You would greatly benefit from writing up such a food plan for yourself.
I'm not necessarily advocating calorie counting, but at its most basic, that is what a weight loss plan is. There are a myriad of websites, Livestrong and caloriecount for instance, where you can input your information. Age, gender, weight, height, activity level. Find out how many calories daily it takes to MAINTAIN 340. Subtract 500 from that, and you have your daily total. Take that daily total, write down the foods you love, and break the calories down into meals and snacks. If you allow yourself 200 for snacks, that is, for instance, 2 medium bananas. Of course, for 100 calories each, there are plenty of other choices available. So you've removed 200 calories from the daily total for snacks, and the leftover you can divide by 3 for breakfast, lunch and dinner.
Nonstarchy veg is a great filler upper, and basically you want to have a balanced diet. Go here and look around:
http://www.choosemyplate.gov/
When you have your plan in place, after some learning curve, you should get feel for the eating part. Try to take as much mystery out of it as possible. Know ahead of time what your meals will be. Find some websites to lighten up some favorite traditional dishes that you like and would definitely crave. Skinnytaste.com is fabulous for that.
If you think you would benefit from, and can afford, a meal structured plan like J.Craig or Nutrisystem, that may be a good diet fit for you. Again, you'll know in advance what every meal is, and that takes a lot of the guesswork out of it, and also, really takes the focus OFF of the diet. It puts you on a sort of autopilot.
Perhaps a liquid diet plan, or Ideal Protein (many people here do that) would benefit you. You have to decide that for yourself.
Basically tho, if you don't have a plan in place, you're floundering. So, all of the above is my thought on that. (I should shut up, right?)
As to the exercise....take a walk. You don't have to walk outside--you can take a trip to a mall and walk around inside it--there are groups of people who do that. You could go to the grocery store (counter intuitive, I know) and walk around just the perimeter. There, you'd pass dairy, fruit, meats and the check out counters--at least that is how they have grocery stores set up here by me. Where I work, I'm able to walk several blocks, all indoors, because the offices are connected to the train station--also, there is a group of wonderful ladies who walk the stairs at lunch time, starting at the 15th floor, down, up one time, and down and out for a 15 minute cool down. There are ways for you to do it--if you want to do it.
It sounds like you're reaching out for ideas, and you're thinking about pulling your plan together. If nothing else, here is some "food for thought". Good luck.