Not Dwelling On the Past
It is clear from various Sefarim and in various places in Chazal that we didn't always have free will in the past.. To quote Rav Shlomo Wolbe, [zt"l]:
The great [Jewish] philosophers established bechira as the cornerstone for the whole Torah.... But from this resulted a common misperception among the masses; that all people actively choose their every act and every decision. This is a grievous error. (Alei Schur, Vol. 1, p. 156)
What, then, is bechira? To answer this question, Rav Wolbe refers us to Rav Eliyahu Dessler's "phenomenal essay on bechira" (Michtav MeEliyahu, Vol. 1, pp. 111-116).In this essay,Rav Dessler describes how the "nekudas habechirah - the point of free choice" is different for different people and in different situations. He explains that bechira is not a theoretical concept that can be applied to any circumstance where a person can hypothetically choose between two options. Rather, it only applies to moral conflicts where the two opposing forces are of approximately equal strength, the person is aware of the internal conflict, and he makes a conscious decision in one direction. When a person does something over which he does not experience conscious conflict, or if the compelling force on one side is significantly stronger than the other, the fact that he is theoretically able to decide either way does not qualify his act as an expression of bechira.
Once we understand that we didn't always have free will in the past, we will prevent the guilt from dragging us down into a vicious cycle of despair and continued falls. And guilt can be even more dangerous than the falls. As they say: "It's not the one cookie you ate that broke the diet. The diet ended when you felt bad about that one cookie, and then went on to finish the entire BOX!"
But when we talk about the present moment, we can never know how much free will we have and we must always try our very best.