Might you be doing it wrong then? Effective practice requires you to stretch far enough, but not too far, lest you become panicked. It's better focused than "general". It requires concentration, effort and introspection.

If you fail completely, you might be trying things too far out of your confidence zone. A mousy introvert isn't going to morph into a charismatic rockstar within a single leap, no matter how much the introvert might want that to be the case. Odds are, he'll seize up and freeze, in which case the only thing he practices is the act of freezing.

Practice is the only way to learn anything, but you can practice in a lot of different ways. Everyone has different learning styles, you should find yours. Don't give up. Shit is hard.

I was pretty shy / introverted in my early 20's. Got a job as a whitewater rafting guide, where I had to interact with new people every day (in a foreign language). It quickly goes away.

Another friend was terrified of public speaking so started doing stand up comedy, and now helps run one of the regular shows in a pub here.

You have to put in the hours to an extent, it's not just a case of trying a couple of times and giving up. Alcohol helps to a degree, but obviously don't go crazy with it. Probably hanging out with extroverts and taking note of what they do helps as well.