From what you said I think the major problem is that the weight gain has made you hate yourself which is turn has made you feel less affection towards your partner and now things are very strained towards you both.
I am not saying that he is not to blame here but the majority of your post was about your weight issues. I feel the same at times and recently I made a change.
I think you should join a gym, get a personal trainer or go too the classes. Start eating healthy, shake diets are crap and they are not for long-term results. I would recommend you download an app called myfitnesspal, it works out your goals and how many calories you should eat a day. Try to get more protein in your diet and less carbs and sugar.
I generally eat scrambled eggs or boiled eggs for breakfast, cereal, you can also have oats with fruit etc. Lunch, soups, sandwiches, pasta, salads. For dinner, I make a lot of homemade healthy curry's, spag Bol, chicken wings, sweet potato, sweet corn. You want to make home made food and avoid processed food. If you cut out all junk, take aways, fast foods from your diet that will be a good start. In the gym try some weight training, HIIT training and cardio combined. If there is a pure gym near you I would recommend you join one and speak to a PT even for just advice.
I think if you start making these changes, you feel feel more happy in yourself, your partner will notice you again and the weight will drop off. Even if in the end you want to leave him, you will still feel good and look good for you. If you want any advice, just ask.