If we're working on something and the initial or main loop becomes annoying or we just get tired of the track in general then we usually determine that it's not good enough and needs to be shelved or fundamentally changed in some way. We only finish a song if it is still dope to listen to on repeat for hours on end while working on it.

We definitely still get tired of hearing our songs after putting in 80+ hours of editing work, but after a break for a week or two, if it doesn't sound bad ass again there is probably something wrong. This is of course assuming you don't have the loop droning problem, which is a different problem, and conversation, altogether.

That being said, if we're just not feeling working on something in the moment, (when it's not on deadline), we'll probably just jump to another project, or jog through until we find one that vibes.

Regarding what OP said about not bouncing WIPs to mp3, I'd assume that's entirely subjective, and mileage may vary for any number of factors. Personally, we bounce everything and put it on our phones after every production session. I listen to our WIPs all the time, sometimes even 50% of my total music listening time each day. For me, it's really helpful for a multitude of reasons: referencing mix in my car, phone speakers, various headphones etc.; scrutinizing over details of mix/arrangement/sound design/etc.; keeping the tune in my head to help imagine and write other parts of the song in my head and, or adding/changing parts to existing sections of the song (inspiration can strike at any time); listening at different times when I'm not in production mode (i.e. maybe it's super annoying after work, or in the morning, or during traffic, or on the other hand, maybe is a great song to start the day, or one to road rage to, etc.). Also, we try to make music that we want to hear, so yeah, I like listening to our own tracks too and if you like your own music, that should probably help too.

I think what helps people stay productive will vary greatly from person to person, but it's also important to accurately identifying what your current struggle currently is. As an analogy, compare producing to socializing. Say someone thinks that they have trouble carrying a conversation with new people. This may be true, but perhaps the reason the conversation is not flowing smoothly is because it started off awkwardly. Perhaps the root problem is that the person needs to get better at striking up a conversation more eloquently so that the conversation flows more naturally, as it would with a close friend. A conversation should be fun, and it should be easy. It's usually easy to talk to close friends. But if it's a new person and the interaction has a rocky start, things wont be so smooth, and the potential conversation may be ended short and not finished at all.

I think songwriting shares some similarities to this model. For some producers, they may think, I have trouble finishing tracks. This may be true, but perhaps the problem is they need to get better at starting good tracks. (Here good meaning that you, the producer, truly likes it and thinks it's good)

Again, a highly subjective topic, but at least for us, it seems a lot easier to finish the tracks that we like the most from their inception.

Also, if you were confused by the interchanged use of "we" and "I" above, it is because I am referencing working in a production duo vs. just what I do on my own.