I either have to talk all about computers, or all about music, or whatever that they're interested in, but never a mixture of everything. The peer groups I see are all so specialized and serve niche interests; none of them are generally interested in everything.

Think on that statement for a little while, and maybe you'll see the source of your trouble. This is kind of a growing up phenomenon. You aren't unique in this regard.

You seem to expect your friends to be a perfect clone of you, with the same interests (you have a few things that you're interested in; you are not a "mixture of all things"). The older you get, the more you'll realize that sharing one or two interests with someone is enough, as long as you enjoy each others company. You may find a few "soul mate" type friends in college, who like the same bands you like, who enjoy the same activities, are nerdy, etc. If you go to a school that specializes in nerdy subjects it's actually very likely, whereas if you go to the local party school, it is very unlikely. So, yes, it may get a lot better in college--but then when college is over, you'll still have to be able to build relationships without that forced captivity thing, so you might as well develop the skill now.

The best way I know is to get involved in something you like. If it's music, start a band. If it's nerdy stuff, start a group that builds crazy nerdy crap and call it art. If it's theatre, join a local company or take classes. If it's a sport, join a league. If it's drinking heavily, start tailgating at football games.

I'm deeply introverted and a loner (and mostly I like it that way), but whenever I've felt the need to expand my social circle I've usually started a band, and right away my circle of friends expands dramatically. I happen to love talking about, listening to, and performing, music...so this works out. If you find that people who only talk about one thing are tedious, this can be a problem...so I guess you'll need to figure out something that you don't mind only talking about for a few hours each week, so that you can do it regularly enough to experience a bond with folks.

Where are the truly interesting people in life?

I don't know. What do you mean by "truly interesting"? Are you truly interesting? Does everyone in your life have to be truly interesting for you to want them as friends? I'm empathetic to your position on this, as I kinda suck at having and keeping friendships alive, too, and it's partly because I find most people tedious after an hour or two of their company. But, I'm old enough to realize that no one is perfect, including me. Some of the most interesting people I've ever met (including a handful of famously interesting folks) are still not people I would want to spend several hours a day with.

There's also something I've learned only in the past couple of years: When a conversation falls flat, it's always as much my fault as it is the person I'm talking to. As dumb or slow or weird or bland as the person I'm talking to may be, there is almost certainly something I can do to make it more interesting for both of us. Maybe it's just telling a joke, and being funny (because being entertaining is as much fun for the entertainer as the entertainee). Maybe it's introducing the person to someone else, and making the pair into a trio or more. pg is hilariously and famously good/bad at this (anyone who's spent more than an hour or two with him has heard the words, "This is the guy you need to talk to!").

How do you stop yourself from over-analyzing your current friendships?

Do something else. If you find yourself thinking too hard about something dumb, like, "Are my friends smart/interesting enough?" Call them up and go do something that you enjoy with them. That'll answer the question, because they've just shown that they're smart/interesting enough to do whatever it is you enjoy, and it'll take your mind off of the wacky self-analysis. Of course, if they all bail on the idea and don't want to leave the house or something, then it answers the question as well...in the other direction. Find more interesting friends who like to do stuff. Doing stuff is actually a really good cure for a lot of mental blocks. I recommend it. It doesn't even matter what stuff. Ride bikes, go bowling, serve soup at the local homeless shelter, build big art, put on silly clothes and golf, etc.