Welcome. I think introverts often have a lot going on inside their heads. That may be why large amounts of people can sometimes drain their energy. Though I don't know if I'd call myself an introvert per se, there's definitely a very shy, awkward side to my personality, and I appreciate alone time.
I used to have social anxiety really bad to the point I basically had no life, I went to a therapist but everything he had me do I could find in a self-help book so I checked a bunch out from the library and (also am on lexapro for depression/general anxiety disorder) I can easily approach people I don't know now or wouldn't have spoke to before and it feels good but I do feel drained more quickly than my very social friends. I'd say I'm an introvert mostly because I will make small talk but it seems pointless to me, I either want to be able to talk about everything with a person or not waste my time when I won't see them again (like meeting a girl at a party.) It just depends on if I click with someone or not how much it drains me. But I keep my private life pretty private and my feelings to myself and my journal not because I'm ashamed of them but just because that's who I am, I don't like to bring up personal things in casual conversations. Like I saw my second cousin who's older and I've met like three times and she thought it was weird I didn't think to show her my engagement ring and tell her...my friends know that I'm crazy about him but I'm not one to run around putting my life on display.