While I am not a psychologist, I can empathize with your issues. I think it is a combination of being still in your 20s, your personality, and a tendency toward social anxiety. Many of my friends who are in their 20s think there is something seriously wrong with them.
Even though it is not your natural personality to be outgoing and appear relaxed and confident, these things can be overcome.
Your problems concentrating are possibly a sign of food allergies, believe it or not. It was for me. I wish getting tested for Celiac disease was routinely done because they are finding 1/130 people have this problem with gluten. You really want to get tested for gluten problems before stopping gluten. There are 3 blood tests they do. But you could try eliminating dairy, nuts, peanuts and soy without consequence if you like. Any foods you reacted to as a kid are likely problems now, but you could have other unrecognized food allergies too.
I don't like to study and excel at procrastinating. I need to go study right now! lol. For me to learn, I have to either write or type out the information to force my brain to acknowledge it. It takes the action to help me learn, but it works.
To boost your confidence, it would help if you felt like an expert at something that you find very interesting. Even better if lots of people think it's cool, but it doesn't have to be mainstream.
I had a battle between fantasies of being in the limelight and being shy because of bullying when I was young. But when I was forced into the limelight, if it was something I worked at, I actually loved getting a crowd happy. Now that I'm in my 40s, I wonder why I was so concerned about what people thought. Who cares if some people don't get you? Other people will, and those are the ones you will choose to spend time with. That's why hobbies and interests are so important.
Right now you need to fake it until you make it. People can't really tell the difference, and they tend to relax around people who are relaxed. And you don't always have to have something witty to say. Sometimes you can look smart just by sitting back quietly and observing. Avoid the worst mistakes of depending on too much alcohol. Stick to one or two drinks so you know what's going on. Some people think it is easiest to drink themselves past uninhibited and into a fog, but you will regret letting yourself get out of control and not remembering big chunks of the night. And cigarettes repulse nonsmokers, so don't fall into that trap.
Improve your nutrition, learn how you need to study to learn, get some good hobbies that you can socialize with, and fake it til you make it. Lots of people feel how you do, and you just need to manage issues one at a time in a logical way.
I should also say that I learned relaxation therapy with biofeedback when I was in my 20s. That helped. Yoga would be very similar, I think. Exercise and working out with weights has boosted my confidence immensely. When you look hot, you will feel confident. All it takes is use of weights and cardio. You don't have to compete, but do care about technique. You can turn heads. It's totally doable.
You might be interested in this info and any other info you can get about adult sensory processing disorder:
( I am not telling you to there I am just saying it has some good info.)
If indeed you have this it is like pulling teeth to get a doctor to listen to you, also if you think you have it you need to be referred to and ocupational therapist that specializes in sensory integration-- they are the ones that will diagnose you.
I hope this helps and I empathize,
I forgot to mention I know one of the main triggers for my migraines are sensory related and do have a sort of social anxiety but that is do to a nervous system that cannot filter input and feeling overwhelmed.
Here are some more sites that will help:
There is even link for a check list.