i don't know how much this will help but i'll try;
years ago i went through the same issue as you with my writing. starting a 5 bar loop, being excited about it for a while then deciding it wasn't good enough and just getting frustrated with the result. It was a very destructive approach, one that lead me to 2 years of trying and failing. I definitely had almost as many little sessions as you, if not more. I wrote 1 song early one day and as usual, started it, worked on it, got excited, then 'tada!' it was absolute crud, i wanted to write something emotive and dark, but all i could put down was lame, cheesy melodies, the bass sounded cheap, the beat was right out of a kids show, but I finally snapped, for whatever reason, i finally dropped the 'idea' of trying to make the song into something and in a moment of rage, said to myself; sod it, maybe im doomed to write crappy cheesy amateur sounding songs, but if i finish this, as least it will be the worlds worst - FINISHED song! after all, i thought, no one has to hear my atrocity.
the reason i tell you all this is just that idea, i don't know if this helps but please, stop listening to other artists on a pedestal, don't compare your work and just focus on finishing them even if its crap, go through the motions regardless and only stop when its a finalised piece of poop. don't listen to ANYTHING unless youre going to do something about it, if youre not feeling the song become clinical, 'ok this needs more X' 'this intro's too long & draggy'. focus on getting something done and dont play back the track unless youre going to do something on it, this will help reduce how worn out you get with a track.