I did a 12 day fast for prayer also when I had a bad patch in my life, around day 6 I got sad and started to break. You need to go outside for a walk and legitimately talk to yourself and pump yourself up to keep fighting the good fight. As soon as you sleep and wake up you’ll be so happy you kept going.
Well for starters you need to know what you are struggling with if you hope to overcome it. I could be 100% wrong (I don’t even know you) but it sounds like what you are really struggling with is that pesky inner voice that we all have, self doubt. That’s the one that sets us up for failure, makes up give up, and stop us from even trying again.
The noise that comes from that inner voice is actually meaningless. I mean the message is pretty good because it picks the exact words that are the most effective but you can start taking it apart by seeing it for what it is; plain old, ugly, and boring, self doubt.
Ignore the message, step out of that moment it is trying to put you in and take a cold clinically look at it saying “oh, this is nothing more than self doubt”. That’s all it takes to start depowering it.
Will you fail in your goal? Maybe, maybe not, but you should make that a separate issue from your self doubt. I used to suffer from self doubt when I was a younger man until I started internalizing how irrational it was. I went from that paralyzing fear, to actually just getting annoyed when I felt it, to pretty much getting rid of it.