You may as well get used to it, because if you write, you're going to get criticism. Whether it's close family or friends, complete strangers or a client - it will happen and it will hurt!
You may have spent hours carefully crafting the perfect piece, only to have someone let rip about a part of it. So be prepared, and start growing the thicker skin required.
So now it's your turn, but not to criticise others - this is your own work. I touched on the "proofreading a day after you wrote it" a while ago and this reinforces it. You need to read through your work once you've forgotten all about it. Go in completely open-minded and treat it as if it was a competitor's work. If it doesn't flow, fix it. If it sounds like a load of rubbish, be honest and rewrite it.
Do not send it to your client until you're happy with it and then, if they criticise it, be gracious and put it right. This is why you need to allow a certain amount of time when writing, it's not the initial draft, it's the edits!
When to STOP!
Ok, you've checked it day-after-day. But every time you read it, you alter something and feel it's a never ending process. You're not sure if it's good enough, what do you do? Send it.
It may go against your principles, but editing is there for a reason. Let the client read it and decide for you, it's their work and they will soon let you know. When it's only minor issues, and you're a bit of a perfectionist, you'll keep altering it until it's no longer the original piece. You're wasting time on something that's probably cock-on in the first place!