• conditions aren't quite right;
• you're not in the right frame of mind;
• whatever you are doing won't meet your own high standards;
• you're afraid that if you can do it once people will expect you to keep on producing to the same high standards;

OK, first things first:

Standards and expectations are subjective. What you might consider easy, someone else may find difficult and vice versa. Brain surgery is not considered difficult for a brain surgeon.

So the next question is - if the item is well within your capabilities - why aren't you doing it and doing it to the best of your abililty? Are you waiting for something or someone to jolt you out of your miserable way of thinking? Or are you waiting for some unseen point in the future? I'll wait until the kids leave home before I...; I'll go on that diet once I've eaten all the "bad"  food we have in the fridge; I'll wait until i've won a million on the lottery before I... or until great aunt dorothy dies and leaves me that house in the 'burbs! Put it like this - you might be waiting an awful long time.

If the item(s) are outside your normal zone of comfort - the next question has to be asked - why aren't you willing to challenge yourself and your thinking? If you are not willing to challenge yourself you might as well go and buy a long box and go and sit in it. The effects are about the same.

It seems to me - that perfectionism is just an excuse - either not to finish anything that you start, or not to bother starting anything in the first place. So what happens? You burn the midnight oil to get whatever it is completed, because you failed to start "it" at a reasonable time. You probably justified your actions by finding other more "important" things to do. And because you managed to get something finished and handed in - OK it wasn't your best effort, well you didn't have time did you....you feel justified in deluding yourself that it is OK to keep on delaying and putting things off, until you can do so no longer. In the real world - extensions just don't happen. Does this form of procrastination serve you any better than standing in a corner with your arms folded across your chest, saying - no, I'm not going to do that - because I don't want to...so there!!

Probably not, at least you would be less stressed (out of work perhaps) but less stressed, but at least you'd be honest with yourself and everyone around you.

So stop dotting the i's and crossing the t's !! and just get on with it. Now is all the time there is, so stop wasting it. It might be a cliche, but it is also true that you only have one shot at today and every day at your life - do you want to get to the end of it, sitting in your long box wondering what might have been.