Which has a greater impact on blood sugar levels: exercise or diet?

I have been diagnosed with high blood sugar for over five years and have not yet recovered to normal blood sugar levels, and I am still battling the condition. I exercise diligently, but I have been struggling because I cannot give up high-carbohydrate bread.

The doctor also advised that only by cutting out bread can blood sugar control be achieved. Finally, with a determined mindset, I stopped eating bread for two days and set a goal of keeping post-meal blood sugar below 150 (when I eat about one piece of bread a day, my post-meal blood sugar is around 155). Today marks two days since I stopped eating bread, and my post-meal blood sugar was 139, as shown in the chart. I need to measure a few more days, but it seems I have achieved my goal for now. Is this level okay? I think blood sugar levels are more influenced by diet than exercise.