Is there anyone else like you out here ? Thousands, tens of thousands, at least...I meet quite a lot of them. 
I doubt if any of my current friends or workmates would ever believe it now, but yes, I know just where you are coming from. When I was at school if I was late for a class I would do anything rather than walk into the room...even now I find walking into a room full of strangers (or sometimes even people I do know) very stressful. Hell, why beat about the bush, frightening is a better word. Parties...let's not go there. Literally !
The ironic thing is, I am a larger than life figure whose reputation goes far before him, I seem to get on with virtually everyone I meet ( one of the reasons I meet so many shy people is they find me easy to interact with) and I am considered a take-charge, get-up-and-go kind of person who is useful to have around. Any time now I half-expect to be nominated as chairperson of a local charitable group...whether or not I would ever have the guts to take the challenge on is something I prefer not to even think of at present.
I get the funny feeling that this pattern of behaviour is called "over-compensation" - I don't care really, it has worked well for me.
Well, the good news is, I was pretty similar to you at age 24. One thing that jerked me out of the recluse trap was getting a job that involved dealing with the public. (Yeah I know, incredible isn't it. 
One of my talents is not picking jobs.) This was what you might call "habituation therapy" - I got used to dealing with people in nice simple routine situations where I just wasn't worried, and so my body unlearned all the nasty bad breathing habits and other anxiety-related physical symptoms. (The shaky voice is partly the effect of adrenalin from the fight-or-flight response, but also largely the result of tensed up rib cage muscles. Breathing exercises like the ones you get for yoga and meditation are good for solving this problem - and the first time it works, boy don't you feel great ! )
That's not to say that you are magically cured by "simply" believing in yourself. I could as well say that you can learn a different language by speaking German; the aim is the process, but what is the technique ? To believe in myself, I found that I personally had to find things that I believed in; rather like a parachutist who doesn't have to believe he himself can fly, he just has to trust his parachute's ability to carry his weight down to earth at a safe speed. Watching others, and learning that it is all about processes, and if you use the same process in exactly the same circumstances then you will get the same effect, is a great way to do this. You start off taking orders in a Drive-thru, start working in a gas station, then end up in a clothing store....
Advantages ? One is to have good role models - people you know well, whose mannerisms you can copy. (Most of "my" popular mannerisms I have stolen from others, because they provoked a favourable emotional response in me.) If you can stop worrying about how you look, and start aiming for a good impression - in other words, replace a negative mental image with a positive one - then that is half of the battle. It's rather like guiding a missile; you don't spend all your time looking at where you don't want it to go, you focus ( or get it to focus) on the result you do want. Mix with a group of people whom you admire, who have positive attitudes and deal well with those they meet, and it will rub off on you. This is not just because they will encourage you (particularly if you ask them for pointers, that's flattering for anyone) but also because you will build up a mental template of what you want and expect to happen; and I'll tell you, the closest thing I've ever seen to magic in this world is the way that your circumstances can respond to your expectations. I suppose a lot of that is down to the fact that the people you deal with will take their cue from you if you seem to know what you are doing. (The basis of salesmanship, when you think about it.) Humour is also worth its weight in gold.
That's dealing with work situations. Social situations...well, I suppose you could say I kind of chickened-out there. I don't have any magic answer for you, never did find one. What I did find was a pretty good work-around; I learned to socialise on the job. And that's how I got into the habit of being so friendly, learned to talk to all sorts of people informally (being formal is the real killer for lots of people, very nerve-wracking, so being very chatty is a great way of putting most people at ease) and gradually learned to spread my wings socially.
Where I've failed to triumph myself is, I still have difficulty going to purely social events - I've probably spent more time licking postage stamps than at parties ! - because I use what I'm actually doing as a way of defining what I'll do. I can do purely social things, but usually only one-on-one; in a group, I find much harder. It takes practice I suppose, and my health is so poor that massive chunks of time get devoured by illness and "pure" socialising tends to be a low priority; there are too many other things I'm missing out on, and I have fun working. So I have ended up with a pretty limited circle of close friends, and no-one will ever call me a party animal.
Aside from reading Dale Carnegie's "How To Win Friends And Influence People" and "Psychocybernetics" by Maxwell Maltz (yes, that's where I first heard of a lot of these techniques ), there are other ways to cope with this sort of problem. For example, there are medications such as SSRIs on the market which are designed to treat "social phobia" as some would call this; Prozac has been a wonderful help to me, for example. The problem there is getting the right one; some of them can have pretty severe side-effects, so you have to keep your eyes peeled for unusual mood swings, excess anxiety (!) etc, and be willing to try a few. (That can be a roller-coaster ride.)
As you can tell, I used both behavioural and in the end chemical help to get round the problems you have. Depending on your circumstances - whether or not you do have close friends or family, whether or not the people around you or your background tends to be negative - you might want to do it differently. Different shoes fit different feet.
Hope this helps, don't hesitate to ask if you have any queries. (This is not a situation where you can avoid all setbacks - but then, I think you've already decided changing things is the priority, right ?)