Hi Ash,
It is not atypical for partners to feel threatened by WLS, especially when they are overweight themselves. Concepts such as "Will she leave me if I stay overweight and she is thin?" "How will I feel when she gets attention from other men?" etc can certainly result in anxiety and negative treatment may be a means to controlling these outcomes. Unfortunately, this stems from low self-esteem, insecurity, immaturity, trust/respect issues.
There needs to be a sensitivity towards your partner as they are undergoing changes themselves. Reassurance is paramount. However, I will be brutally honest. You cannot promise what you do not know. Many people who undergo a major weight loss go through radical physical and psychological changes, in essence, they become new people. What you are envisioning as your future today, may indeed be nothing what you are envisioning.
The focus should be on the current moment. What is your goal, to lose weight I'm sure. What is supporting or sabotaging this effort? The focus needs to be on what you need to be successful. It already sounds like your inner voice is telling you that he is hindering your progress and/or your ability to enjoy your journey. Boundaries must be set and self-love needs to be your priority. You've come too far and sacrificed too much to fail.
On a side note, my ex husband and I were banded together in 2009. We divorced in 2017. Referring to paragraph 2, there was no way I could ever imagine the extent by which I would change. I lost 120 lbs and you bet it changed how I looked at myself. I not only had a higher esteem, I had a desire for all aspects of my life to be healthy. I am not claiming the divorce was solely based on weight loss or surgery. Rather, the weight loss experience was empowering and I wanted the best for myself. I had a new perspective on life and knew I could be happier. Fastforward to today, I am remarried to the most remarkable man alive and my life no longer feels imbalanced on any level.
I hope that you are able to communicate your feelings and needs with your partner and come to an understanding that support is needed and boundaries respected. If not, I wish you the strength to make the necessary decisions to make you happy and healthy