Why is my bread dough sticky?

I'm been making this Japanese bread "Melon pan" many times and it had been success.Then my brother told me to add more sugar to make it more sweet. Try adding little at first, the bread still taste the same. So decided to put a lot more sugar. I started to knead my dough and it very sticky, already added flour (even too much) but still sticky. Continued to knead it for 15-20. Still Sticky :( But somehow I baked it and my brother tried it and said the amount of sugar put in was good. But the bread lost its crispiness so It was a fail. What can I do to solve this problem? Thank you

This is the original recipe that worked before I added a lot of sugar.

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  Bread Dough - 140g Bread Flour (4.94 oz) 25g Sugar (0.882 oz) 1/3 tsp Salt 5g Non-Fat Dry Milk Powder (0.176 oz) 3g Instant Yeast (0.106 oz) - a little less than 1 tsp 1 tbsp Beaten Egg 70ml Warm Water (2.37 fl oz) 15g Butter (0.529 oz) Bread Flour for dusting

  
  


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  Cookie Dough - 25g Unsalted Butter (0.882 oz) 35g Sugar (1.23 oz) 25g Beaten Egg (0.882 oz) 80g Cake/Pastry Flour (2.82 oz) 1/4 tsp Baking Powder Bread Flour for dusting