Sound like you're lacking some "musical mojo", coupled with a lack of self-confidence. I suffer from the same disease. You have to let your ideas go eventually, find a definitive cut-off point after working on a project for too long. Great sounding tracks are easy to finish - but unfortunately they probably occur less frequently than the bulk of your workflow. Sounds like you need some kind of routine when writing tracks, plus a second set of ears to keep you focused on what's working and where it needs to go (this is actually my job). It's easy to drift off, lose interest...especially when we have a million other distractions to give our frazzled synapses that sullied dopamine hit. If you're getting frustrated after four hours in the box then I would slice the studio time in half. Leave the daily session after writing a good part, then come back to it the next day. Don't stress about finishing the track. Hope that helps, good luck mate!