It's actually a lot simpler than you might think, take out your opinion for a bit.
Just keep on trucking! :)
Seriously, the best advice I've ever gotten was something like this: 'postpone your judgement'.
Procrastinate for a bit, do some dishes, fold your laundry, go outside.
Get back in the studio and work on a different part of your project, come back to the 'difficult' part a day later, you'll see it wasn't a problem after all. Most of the time.
Oh and google: Oblique Strategies. Great stuff from Brian Eno an Peter Schmidt.
Or read some Edward de Bono, he has some great insights into these matters.
Ok, I just experienced a very simple example of why focusing on other stuff tends to help.
Yesterday I was working on an animation and I couldn't get the sound of trash can to fit the image. It was a library effect and it just didn't fit in.
I stalled and got a bit pissed of. I considered pitching it, but then decided that would be too much of hassle. I know from experience that you can fiddle with such things for ever.... So I stopped working on it and went ahead on with other events on the timeline.
Today I answered this question and when I was thinking about my answer an hour later in the bathroom (just doing the laundry ;) I noticed that I actually own a little metal trashcan. Totally forgot about that! And you know what, it has the perfect squeak and resonance! Perfect fit!
Lesson learned= take a step back, focus on stuff that you can do!