What is the number 1 limiting factor of fatloss?

Diet is the hardest part for me. After 2-3 months on low-carb diet even the worst junk food appears as if it has landed from heaven and not even exaggerating.
My physique is more of a gainer, can pack muscle mass and fat fairly quickly. So my body doesn't like loosing weight. I have to go ballistic on calorie deficit to get the fat burning started.

I don't diet until its prep time but imho Diet is the hardest part. Just my 2 cents

For me these are genetics and diet. The first time i had to shred was the hardest because the layer of fat under my skin was thick.