Hi Emily, I hear your struggle. I am a T1D with a "normal" BMI of 23-24, but until about 9 years ago I struggled with weight. I have long reacted to higher carb meals with weight gain and high blood sugars, so I decided to try low carb back in 2017. That's been quite helpful to maintaining my weight where I want it, since higher carbs require more insulin to keep sugars down. But that's not the only thing I've done...
Since I also noticed insulin resistance at the same time (needing more insulin with no diet change), I had my thyroid checked and learned that I had low thyroid and needed a bit of medication to normalize it. I regard this medication (Cytomel/T3 for thyroid) as having been VERY helpful in correcting my low thyroid along with the related insulin resistance and weight gain.
Could low thyroid be an issue for you too?
I also use some other things that are helpful too: insulin mimetics like Alpha Lipoic Acid (a supplement) have helped my body be more insulin sensitive, so I use less insulin. And lower insulin requirements mean less weight gain. Other kinds of insulin mimetics would include Berberine, which acts like metformin in making people more insulin sensitive, but can be useful to T1Ds.
There may be other things going on for you that you haven't mentioned, like excess stress or lack of sleep or sickness, that can make us less sensitive to insulin and more likely to hold onto weight. So, if you can think of these other things, please fill us in.
Also...some resources I've relied on: Low Carb Down Under has some great videos on Youtube, if you want to explore low carb. And Dr. Berg on Youtube is also very supportive of T1Ds with low carb.
Best wishes!
Sandra (T1D, 1986)