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 ...
Typically, I'll drop a marker and keep moving forward. On most projects that I'm on I don't have the luxury of spending hours on a specific sequence, let alone a particular effect. If it's not coming to me fluidly at that moment, I keep making progress on the rest of the project. Eventually something, a clip down the timeline, a phone call, a daydream, ...
Well, that depends on the client. Is the client happy with what you consider 'second best'? If so, make the deadline and quit struggling. Repeatedly missing deadlines even if your work is amazing and perfect won't win you much more work.
Does this happen often? Maybe you need to re-consider your process and spring for some gear or SFX packs that will ease ...
As mentioned above, I avoid and come back later. Sometimes you're clutching at something so hard you don't realise it's slipping through your fingers. Go away and tackle it later. Normally it will just 'click' with time. And if time has gone and it needs to be submitted, I go with what I've got. I guarantee what's sub par to you, is fine for your client.
I ask advice of my friends, sometimes I buy new source material to work with or plug-ins to process with...but, essentially, I just keep plugging until I find it. This is the fun experimental part of sound. Sometimes you find the sound you had imagined is not even as good as a completely different sound you created while struggling.