How do you use them? I just bought some of the Avent ones to use the ventilated ones as have sore nips and the pressure of my bra is irritating them. But just read the instructions and it says they shouldn't be worn for longer than 40 mins at a time as they may put pressure on the ducts. So what's the point? Surely you need to be able to wear them all day to relieve nipple soreness from clothes? (I get the point of the unventilated ones to collect milk, but it specifically says the ventilated ones are to relieve irritation)