I bought 4 brass mouthpieces and like many other people I found they were an incredibly tight fit. But once they had been on for a few hours the corks remained compressed for a short while after the mouthpieces were removed, and the mouthpieces were easy to put on & remove at that time. So I decided that after I'd cleaned the mouth pieces & neck pieces I would re-grease the corks and put the mouthpieces back onto the neck pieces and then put them into a small case where they would be safe. They might be in the case for a week or two before I need them again but the metal mouthpieces are extremely easy to put on & take off at all times now.

So if you're finding that a mouthpiece is incredibly tight to put on then try leaving it on instead of putting it on only when you are about to use it.

But make sure you re-grease the cork every week or two so that the mouthpiece doesn't get stuck on, as it possibly would if you left it untouched for months.