Need help with dietary changes for prediabetes

Obviously my doctor recommended diet/lifestyle changes with blood tests showing I’m prediabetic, but I’m very confused about how to change my diet. I’m wondering if anyone else has had issues being on a highly restrictive diet? My diet has been gluten, dairy & soy free, very low sugar intake (only natural sugars), very low sodium intake (no added salt). I have sensitivities and extreme pain reactions to all of those foods, but I’m also chronically underweight and my diet makes it really hard to eat enough food in a day. I only had an increase in my glucose levels over the last 6 months but it was somewhat significant. Any recommendations on improving my diet from here?? Should I try to re-introduce anything?