At my heaviest measured weight (immediately before I got serious about weight loss and a new healthy life) was 410 lbs. I highly suspect I was as much as 425 lbs at one point. In Sept of 2014, I broke the fibula in my left leg down near the ankle I had a really hard time getting around at that weight (410+). I got fed up and and as soon as that damn leg healed I started. Today I am 332 lbs and I have no intention of stopping anytime soon or ever as this is a life style change. My goal weight is 250 lbs and then I will reevaluate to decide if I need to lose more. I will adjust diet as needed when I decide to maintain.
I was not quite as heavy as you, but I was damn heavy. I think I can give you relevant advice as I have been at least close to in your shoes.
Stop restarting. You said you will lose, then get frustrated when you stall and quit. You have to stop this. Remember that this is not a diet, it is a life style change. Do not stop eating healthy just because the number on the scale did not move. When you falter and gain a little, you MUST not let that stop you; you must become that much more determined and you MUST bounce back.
As big as you are the exercise will literally HURT. Back, knees, hips, you name it, it will hurt. Exercising at that size if painful, but you must push through it and eventually it will hurt less until one day you realize it no longer hurts. I recommend walking and, if possible, lap swimming (not because it is a great weight loss exercise, but because it is low impact). Just do whatever you feel like doing in a day. If it 15 minutes of walking, fine; if it is 30 minutes, great. The key thing is to do it 6 days a week no matter. No excuses, do it. You will get past the pain. You need to push yourself on days that you feel like it so that you improve as much as possible.
The diet is key. Without proper nutrition you will perform poorly in your workouts. You need high protein, low to moderate COMPLEX carbs (no simple carbs), low sodium, low sugar, low fat. When you are too low on energy, increase the complex carbs a little (e.g. have oat meal in the morning). Cut the cheat days as much as possible. If you can only have a cheat meal once every 2 or 4 weeks that is better than every weekend. You are just setting yourself back every time you cheat. You can lose 6 lbs in a week then gain 4 of it back in one damn meal; it ain't worth it.
You need a support team. What ever obstacles are in your way, you must find a way to eliminate those even if it means asking help from a loved one. For example, if you don't have time to prepare meals, you gotta find someone to do it for you. That was my biggest obstacle, I flat did not have time to do a 40 hour week job with a 1 hr commute everyday, plus cook 3 damn meals and go to the gym. It wasn't happening. I am lucky now because I have wife to help me. She cooks and keeps my gym clothes ready so I don't have to worry about those things. This gives me the extra time I need to make it happen. Consider a personal trainer. Whatever it takes to eliminate those obstacles.
My typical day (6 days a week) as of today: wake up at 5 go to gym workout for about 1 hr 15 mins. 3 days a week it is heavy weights because my goal is to gain muscle. The other days I do HIIT. go home protein shake shower oat meal with blue berries go to work at noon I have lunch number 1 which is usually grilled, baked or roasted chicken (low sodium) and some veggy (broccoli, etc) around 2:30 pm I'll have a second small lunch (about the same as first). go home from work about 5:30 pm. have a lite supper with no carbs. through out the day drink at least a gallon of water. The water is critical.
Sleep is critical. You must get enough sleep in order to be able to keep this routine up. Your body needs rest when you exercise.
Starting out my routine was obviously not this intense. It looked more like: wake up at 6 go to gym do 15 to 30 mins on the tread mill at 2.5 or 3.0 MPH. go home, go work eat same as above; basically the same as above except exercise.
You can do this man, you just gotta set your mind to it and get all your obstacles removed by getting people around you who can help you on board. Remember, no matter how hard it gets, no more quitting and restarting; those days are over ok?
If you have any questions or need anything, I will be here for you. I want you to succeed. I will help you succeed if I possibly can in any way I can.