If all of the time you're having to scrounge up the will to keep working on something, then don't — simple as that. Put the kit back into its box, put the box back into your stash, and find something in there (or in a shop) that you do fancy building.

I've got dozens of part-finished models in their boxes (and some out of them), some dating back up to thirty years or so. Technically, I intend to finish all of them, except for the very old ones where I now consider their unfinished state to be historical, but I doubt I ever will. And I don't mind — I build models to enjoy myself, and if that's not happening then why would I keep at it?

Of course, you sometimes hit a rough patch that you have to get through on any given kit before it becomes enjoyable again. In that case it's a matter of biting the bullet that you keep putting off, though. For example, I don't enjoy working with two-part epoxy putty, so I've been putting off finishing my M4 (105 mm) tank because all that's left to do is to sculpt a cover over the machine gun. And I did the same when I had to make the cover over the gun mantlet and the sandbags on the rear deck for it. But eventually, I just got out the putty and made those things. However, if you find that pretty much nothing about the model is making you want to continue working on it, then just put it away for later.