Purely to lose weight, in the end it's down to calories.
Bariatric surgery causes huge weight loss due to calorie restriction. Even if you were to eat nutella and milk shakes you would get so sick and would not be able to digest much anyway due to smaller stomach.
Low carb absolutely works if you get into that stage were you dont eat that much (ketosis). You naturally suppress hunger so your calories are overall reduced. As an aside benefit it also helps with regulating hormones and great for skin and hair. As you wont be eating much carbs your body will get rid of water so you will look less bloated and puffy in the process.
Slimming world works if overall your calories are low which can be achieved if you stick to speed food and third plate veg rule. Ditto weight watchers. They are more flexible around traditional family meals and a choice if you cant live without carbs (by carbs i mean rice, pasta etc).
Exante and other shakes, 5:2, 4:3, fasting.. all ultimately reduce your calories to.dofferent extents.
I'd start by looking at whats your biggest motivation, whats your downfall? What caused the weight gain? Really dig into this so its not enough to say too much chocolate, ask yourself why have i been having so much chocolate, im bored/emotional/whatever. Then ask yourself why again and plan how you will do things differently this time and what methods or ways you can apply to get where you want.
Remember, madness is doing the same thing over and over again expecting different results.
Also remember, it takes time and persistence. 3 months seem a long time but 3 months ago seems like a couple of weeks ago.