stop p*ssing about on the forum ;-) kidding, i think we all suffer with this from time to time, especially if it's not going as planned. hmm, i think discipline is the only thing, don't allow yourself to start anything else.

Having said that, sometimes it's good to work on something else & come back to the original image with fresh eyes, nothing wrong with that, as long as you do go back to the original image.

helpful ain't i? :-P

Creativity is often like that.  You start on something with lots of enthusiasm and get a fair ways along.  Then you reach a stage where the creative juices stop flowing and you get tired of what you are doing.  What was an obsession has become a chore.

Ultimately, you'll have to push your way through the chore phase and finish what you have started.  But it does help to step away from what you are doing and let your mind rest and the creative juices gather.

If you want to stay focused on the artwork you are doing, then put it aside and work on variations of what you started.  If it is digital art, make a couple of backup copies of what you've done and play around with them in a completely different direction.  You may find that the variations are enough to keep you going.  They may also be what you need to get back to the original concept.