I can relate to that. I'm new to RP-ing compared to many people- I've roleplayed before a few times with friends here and there, but role-playing with people outta that circle was really out of question.
Though, it frustrated me that as time went by, people began to "grow" out of it, referring to me as immature and childish when I would ask to rp something.
Honestly role-playing has always been so fun for me. It's like a release from stress, and it gives such a good feeling. I really like it, and it's so fun. It's like being a character in a book- or playing a character from a book in a movie or something. I have a huge love for books, and animes- and acting is one of my favorite hobbies. I am one of those people who act in front of the mirror- it may seem childish to some but i genuinely enjoy it.
Deciding that it was time for a change, I went from site to site- until I FINALLY found Umbra- at first it was a bit awkward, but people here are actually so nice and chill. They're willing to be patient, and that's really sweet of them.
So don't worry. I'm sure everything gonna work out for you! Within time, you'll feel comfortable enough.
(BTW I love the vampire x human concept too-)