Willpower is a limited resource, at least for most humans. In my case, I can only resist so much temptation of certain types. Put me through stressful situations with dark chocolate available, and I will eventually eat some. Assuming free will exists, this makes the issue more complicated.
Determinism and free will are not necessarily incompatible, depending on how you define free will. If I'm hungry, suppose you offer me two pizzas, one of which is sausage and mushroom and one of which is veggie that includes green pepper. I will, of my own free will as I experience it, eat slices of the sausage and mushroom one. Is that free will, determinism, both, or neither?