Maybe I'm just weird here - but nothing you've talked about seems inherently weird or wrong to me. Being shy in a group is one of the most common things teenagers experience it, and often times, it's very common in adults as well. It's simply one of those things that people have to grit their teeth and fight through, or be happy with the way they are. Only you can know what's right for you, and what's not. Are you ever going to be comfortable in groups? Do you even know that answer? If not, why not practice for about a month at pulling yourself out of your shell (it can be done, but it will be hard), and getting comfortable in groups? Who knows, maybe with time, you'll get the hang of it, and actually embrace that lifestyle.
Or perhaps you're one of those people that will just simply never be comfortable in groups. Is this necessarily a terrible thing? Maybe, maybe not. But if it's the way you are, then it's simply the way you are - and if you're happy being that way, then who am I to tell you it's wrong? Who is anyone else to make you feel like it's wrong?
Basically, do what makes you happy, but don't rule anything out. Explore the angles, and then see if you're more comfortable, more happy, more whatever with those angles. The ultimate goal should be your happiness - and if you're happy being slightly introverted, then you've already gotten there - society is what is messing it up for you.
Now, the second part is pretty simple, really. Your friends are your friends for a reason. They aren't your friends because you're introverted. They aren't your friends because you're shy. They aren't your friends for any other reason other than that they enjoy being around you, and they enjoy you as a person. Don't overthink things.
People will always intimidate you. That's just part of life. The trick is to realize that while you notice their qualities, they also notice yours. Simply because your intimidated doesn't mean you don't display great qualities that attract people - because if you didn't, you would've never made these friends to begin with, ya know?
Your friends respect you as an individual, just as you respect them. Don't lessen yourself ever. Sometimes people are strong within' themselves, and they end up talking themselves into sadness simply because they don't live by what society may call 'normal' - don't let yourself get pulled into that odd mindset, because it'll get you no where.
I can't tell you that they will never leave you. Sometimes relationships fall apart. I can, however, tell you that if they fall apart, then they can be replaced, and you can replace them, because you've attracted friends once, you can do it again. You're obviously more likable than you give yourself credit for, why don't you start giving yourself the credit you actually deserve? I sure don't see a reason not to, other than you being silly and overthinking situations. ;)