I think as hard as it is, this is one of those times where you have to focus 100% on getting YOU healthy, because the best thing for your child is to have a healthy mum. I personally don’t believe there’s any room in a child’s life for any conversations about “good” or “bad” foods, nor do I think food should ever really be a focus in a child’s life in general. Children copy their parents behaviours and mindsets, so focus on being a good role model rather than what others do. My parents were very matter of fact about eating- food is food, we need food, we all ate the same meals and there was never any “you have to eat all of x before you can leave the table/have dessert”, no tv while eating, just eat mindfully and stop when you are full. Yes we had grandparents who spoilt us rotten and would offer us crisps, chocolate, sweets etc every time we were with them, those were treats and that’s fine because our eating habits were built off of what was modelled at home. Them giving her biscuits when she is there isn’t going to create a “fat” child. Your child has 21 meals in a week, plus probably 14 snacks (assuming one morning one afternoon for example), that’s 35 “meals” per week, your parents providing biscuits for 3/35 does not create a “fat” child- whatever you provide for the other 32 could.
Also- tell your partner. The easiest way to make sure you won’t stick to something is to keep it a secret, because the easiest person to let down is yourself, and secrets hold power over you. Open up, all cards on the table, he can help hold you accountable and support you in changing. If you are serious about change, this is one big step you must take.
Also exercise- my sister and I are and have both always been very active. Not because it burns calories, or to “earn” food, but because we were raised by parents who taught us that we are very lucky to have bodies that allow us to run, cycle, walk, jump, and that pushing ourselves to be fit is a gift not a chore. Try to adopt this mindset. There is someone somewhere who would love to be able to walk for 100 meters.
Spend your time and energy getting your own eating/mindset right and the rest will work itself out honestly. And kindly- if you can afford what you have mentioned then you can afford counselling, it’s just priorities x