I can't finish an entire piece and I'm lost what to do.

Any full illustration that takes hours and hours to finish is almost impossible and I'm not sure why. I've cut all distractions and there isn't any interruptions, but I just mentally second guess myself and do nothing even when I still make an attempt anyways.

I can finish some art, but these are only small pieces that are short to do. I just want to find a way to train my brain to focus and to dedicate to more stuff that requires higher detail and effort put in. I've tried recommendations like timers and small breaks, but still nothing.

I need to keep having focus, but I just don't know how.
I'm a total loss on what to do anymore /ic/, any tips would help me save me a lifetime