I never had doubts ever regarding finishing as I know one can achieve much more than it feels like in the beginning. But then again this might not be too representative as I used to be an experienced mountain/wilderness hiker.
I do sometimes have doubts I might not achieve my goals – I always set goals such as miles per day or reaching a certain place in a certain number of days, I love these minor challenges and it happens automatically inside my brain
I cannot stop it.
Now, after Covid and in the middle of a frustrating and lengthy aftermath, I do fear there will be more suffering and pain and that I might be even weaker than I planned. But still, failure is nothing I mentally consider. It does not mean it cannot go wrong of course. I could get sick, I could get injured. But those are exceptions I cannot predict hence they are irrelevant for me.
I guess your doubts are mainly mentally. My brain works along the lines of Yoda's saying "Do. Or do not. There is no try." But once in that mindset there is the danger to overdo. Forcing yourself into too much during the first week or weeks might result in injury and destroy the remaining weeks. So I would extend to:
"Do. Or do not. There is no try. But do wisely."
Keep in mind, after a local walk that not went so well, that even when out there on the Camino there will be bad days and good days and you might be a better walker in the second half than you were in the first. The body adapts and changes when you hike long distance. If you do not overdo it in the beginning, you might feel stronger later on the way.