There’s a well known adage that if Hashem gave us a test, we must have the ability to overcome it as well. R’ Tzadok HaKoehn says though, (in Tzidkas Hatzadik) that this is not as simple as it sounds. It is true that we all have free choice to do what Hashem expects of us in this world, over the course of our lifetimes. However, in the process of our journey, there are many times when a person is considered an onus.
After the sin of the golden calf, the Midrash says that Moshe said to Hashem, if a father gave his son gold and sat him down on the doorstep of a Beis Zonos – “ma ya’aseh haben velo yecheta? - What can the son do and not sin?” In other words, we find in Chazal that there are times when a person may not have full Bechira.
See also the Rambam Hilchos Issurei Biyah 1:8 – “for the Yetzer and human nature forced her to want,” and see Tosofos in Sanhedrin 26b where they discuss how someone suspected of illicit relations may still be a Kosher witness, since it could be that his desires simply overpowered him. And see the Gemara in Brachos, 32b: “Asher Hari’osi” where Hakadosh Baruch Hu acknowledges to Eliyahu Hanavi that He was the one who had turned the Yidden’s heart away from him.
The Steipler too, in regards to a specific behavior that someone had difficulty controlling, writes: “He is not a Ba’al Bechira now in this area, and the only thing he can (and should) do, are Tikkunim that will help him over time.”
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!”
And even if we may have had some freedom of choice at the time we fell, it could be that we had very little. The sins we did are only judged according to the circumstances and the level of free will that we had at the time. Only Hashem knows if we could have done better or not.
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.