＞＞They get their energy from being social.
I love them, but at the same time, I feel uncomfortable being around them.

I feel you. Listening to them is sometimes hard. Why do they talk about such irrelevant things? Parties are just habits and many of them are a waste of time in the first place. Going home and reading books is more comfortable for introverts. Just avoid boring parties and don't be afraid of losing extroverted friends, which make you uncomfortable. You can have your way. You spend your own time, not get others to spend it.When you are at the party, you can look at them as a movie or a play. They are boring actors. You can be an audience who criticizes without speaking. It's an observation of people. It's not bad.

>>When I m face to face with each one of them, I like her very much. but when they gather, they change their way which makes me feel a bit left out. 

Yes, I understand. They are trying to adapt to the others to make a funny play.  I think they must feel like you. Just try to talk to one of them about your feeling if you have a chance. She/He might say, "Oh, same. I was thinking as like you." Then you become her true friend. 

I'm an introvert on every level. Yet, I teach here on Italki, volunteer in my community, and lead (with great reluctance) a fairly active social life. How?

1.Think outside yourself. Think of others. And I don't mean that in a harsh way. Find someone you can help. It's easier to come out of your ''shell'' when you are helping someone. Teach someone a skill.

Perhaps listening and self-control is something you can teach your extrovert friends. Perhaps you can help someone learn your language, learn how to cook, or any other skill you excel at. 

2.Don't limit your friends to people your own age or with the exact same interests as you.

3.Conversation is an interchange. People love good listeners. So if your shy, remember that you don't have to ''carry'' the conversation. If you do chime into a conversation, talk about something you know a lot about, perhaps a hobby, art, or your favorite movie.

4.Don't force yourself to interact with people you really don't want to be around. People will clearly see how uneasy you are. 

5.Talk to one new person each week. Listen to their problems, opinions, ask them how their day was. Master the art of "small-talk".

6.Schedule your social life. Choose one day each month you want to do something social. Mentally prepare yourself. After each social interaction do something you really enjoy like read a book, cook your favorite food, etc...

7.People are selfish. They love talking about themselves, their problems, their skills. So let them! There's nothing wrong with being a good listener. People will appreciate that you care enough to listen.

8. If you are at a party, always remember you can leave anytime you want.

9.If you go to a party or another social event: Make sure you get enough sleep/rest before you go. Being tired makes socializing worse. Make sure you eat also. Being hungry can cause you to become irritable.

10. Just be yourself. : )

I can somehow relate. I'm an ambivert: I could be both an extrovert and an introvert but I have a tendency to be more introverted with people who are very outgoing, extroverted, talkative, sociable and most especially with those who have a very strong personality or those who come on too strong

I notice that I can be (very) quiet with my American and Spanish-speaking friends and acquaintances. In your case, I would say that you feel uncomfortable with your Spanish friends due to cultural differences. I respect each and everyone's differences. It's natural for Japanese people to be more introverted/shy/reserved while it's more natural for Spanish-speaking people to be sociable and animated/vivacious. Of course there will always be exceptions to the rule

Extroverts and introverts alike make the world an interesting place. You don't have to be like your Spanish friends. If everyone in the group is extroverted then nobody would listen, and if everyone in the group is introverted then the atmosphere would be uptight because nobody would break the ice. 

In fact, being introverted is more advantageous than being extroverted because introverts are intelligent and observant. Plus, it's usually fun to converse with introverts because they are sensible. Silent waters run deep

You don't have to be with them if you feel uncomfortable with them. However, it would be nice too if you would be with extroverted people from time to time because realistically speaking, you cannot be with fellow introverts all the time. Being cliquish is not really helpful in a global society