Introversion is something I feel a lot of people talk about, but they don't truly understand unless they're living with it. Try as they might, extroverts just can't understand why we can't hang out two nights in a row or why we shut up in groups of people. I love my friends and family and the effort they put toward understanding this, but I can tell that they just don't get it sometimes. What I've learned recently, though, is that that is okay. They're trying, and that's all that matters, because I'm still learning and trying too. I can't expect them to understand what I'm feeling when I don't even fully understand it sometimes.
Which brings me to my next point: how to manage it. Introversion is different for each person, so it's all about figuring out yourself and your limits. Some people can handle the occasional loud, crowded party. Others can't. Some people can hang out in small groups more than once a week. Others struggle with making it through just one hang out session. You have to figure these things out for yourself. Figure out how much you can handle and how often, and let the people in your life know. I promise, if they truly care about you, they'll understand or at least make an effort. Never take those people for granted.
Next, you need to figure out how you recharge. This is the part extroverts seem to have the hardest time understanding. They can bop from one social event to the next with no problem. We can't. It's like running a 24/7 marathon if you don't take the time to recharge. The toll is physical and mental; it drags you down in the worst way possible. Do yourself a favor and take the time to figure out the best way for you to recharge. Do you just need a night to yourself with a good book or Netflix? Or do you need a few days without human contact so you can find your center again? Whatever it is, do it. Your mind and body will thank you.
Lastly, be patient. Don't be hard on yourself. You may feel like you're letting your friends down when you turn down an invitation, but your mental health is far more important than making everyone else happy. Your true friends will understand this and still love you even when you revert into your shell.