What Does Self-Confidence Mean in Islam?
Question

How can a Muslim who is really lacking self-confidence become a more confident person? He has tried so many things but has yet to overcome his nervousness when talking to people. May Allah reward you with good for your effort.
Praise be to Allah.Meaning of self-confidence in Islam
Self-confidence is an acquired characteristic that the Muslim needs to know how to acquire so that he will be one of those who have self-confidence. But first of all he has to differentiate between self-confidence and arrogance. 
Self-confidence means being aware of what Allah has given you of good characteristics and striving accordingly to attain that which will benefit you. If you misuse it, you will be filled with arrogance and self -admiration , which are two destructive problems. If you deny those blessings that have been given to you and the good characteristics that Allah has blessed you with, you will become lazy and apathetic; you will let yourself down and lose the blessings that Allah has given you. Allah says (interpretation of the meaning):
“Indeed he succeeds who purifies his ownself (i.e. obeys and performs all that Allah ordered, by following the true Faith of Islamic Monotheism and by doing righteous good deeds).
And indeed he fails who corrupts his ownself (i.e. disobeys what Allah has ordered by rejecting the true Faith of Islamic Monotheism or by following polytheism, or by doing every kind of evil wicked deeds).” [Ash-Shams 91:9-10] 
It is worth pointing out something important here, which is that for the Muslim, having self-confidence does not mean that he does not need the help of his Lord, and it does not mean that he does not need his brothers and people in general to advise him and help him. This is what the Prophet (blessings and peace of Allah be upon him) asked his Lord for, which was: not to be left to himself (or to his own devices), not even for the blink of an eye! 
Abu Bakrah (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “The supplication of the one who is in distress: O Allah, for Your mercy I hope, so do not abandon me to myself even for the blink of an eye. Set all my affairs straight, there is none worthy of worship but You.” (Narrated by Abu Dawud (5090); classed as sound by Al-Albani in Sahih Abu Dawud)
Al-Nasa’i (10405) narrated from Anas, in a Hadith that was classed as authentic by Al-Albani in Sahih An-Nasa’i, that these words are to be said morning and evening. 
Shaykh Al-`Uthaymin (may Allah have mercy on him) was asked: 
What is the ruling on saying “So and so has self-confidence” and the like? Is this contrary to the Dua narrated in the Hadith, “do not abandon me to myself even for the blink of an eye”?
He replied:
“There is nothing wrong with that, because what is meant by saying that somebody is self-confident is that he is certain about something and is sure of it. There is no doubt that things may sometimes be attributed to a person on the basis of certainty, and sometimes on the basis of probability, and sometimes on the basis of doubt, and sometimes on the basis of what is most likely to be the case. If he says, for example, “I am certain about that” or “I am certain of myself” or “So and so is certain of himself” or “he is certain about what he is saying”, what is meant is that he is confident about it. There is nothing wrong with that and it is not contrary to the well-known Dua, “do not abandon me to myself even for the blink of an eye,” because the person may be certain of himself by the help of Allah, based on what Allah has given him of knowledge, ability and so on.” (Fatawa Islamiyyah, 4/480)
How should a Muslim attain self-confidence?
Some of the things which we think will increase the Muslim’s self-confidence are:

Putting his trust in his Lord, depending upon Him and seeking support and help from Him. The Muslim cannot do without his Lord, may He be Exalted. As we mentioned above, self-confidence is an acquired characteristic, and the Muslim needs help and support from his Lord. The more he puts his trust in his Lord, the greater his self-confidence will grow and reach the highest level.

When Musa (peace be upon him) and his people fled from Pharaoh and his troops, and the two sides saw one another, we see the trust that Musa had in his Lord. Allah says (interpretation of the meaning):
“And when the two hosts saw each other, the companions of Musa said: ‘We are sure to be overtaken.’ [Musa] said: ‘Nay, verily, with me is my Lord. He will guide me.’” [Ash-Shu`ara’ 26:61-62] 

Finding out what his strong points are and increasing them; what he does well and developing it; and what his weak points are and trying to deal with them. 

In order to develop self-confidence it is essential to look with gratitude at what Allah has given you of skills and characteristics, so that this will motivate you to strengthen your self-confidence. With regard to your weak points, you have to try to deal with them and develop them so that they reach the same level as your other, good skills and characteristics. 

It is very important for the Muslim who is seeking ways of increasing his self-confidence not to repeat negative words, such as saying that he has no self-confidence or that he will never succeed at work. 
The Muslim has to set specific goals for his life and check on the outcome time after time, because the person who is self-confident will make sure that his goal is achieved on the basis of good planning and with the help of his Lord.
The Muslim should seek righteous companions, because that will give him a motive to succeed and encourage him to make greater efforts. The righteous companions will not overlook their friend’s weak points; rather they will guide him to follow the better path. Thus good companions are one of the factors of success for the confident Muslim.
He should not be distracted by previous difficult experiences and past failures, because that will spoil his efforts and make him look down on the success he has achieved. This is not something that the Muslim wants for himself.

How to control anger in Islam
Controlling one’s behaviour and actions is something that the Muslim can do and it is within his capabilities. Part of that is anger; the Muslim should be confident that he is able, with the help of his Lord, to rid himself of the evils and bad effects of anger and strive to mend his ways and discipline himself to adhere to the laws of Allah. This is something that is very easy indeed for the one who wants to achieve it, so long as he has a great deal of resolve to complete what he wants to achieve of disciplining himself and purifying his soul.
The one who wants to rid himself of anger has only to hasten to take action. This, in fact, is what we are lacking. The words are many but the actions are few. So let the Muslim who wants to purify his soul train himself to take action, do what his Lord has commanded him and refrain from what his Lord has forbidden him to do. Thus he will be one of the successful, In Sha Allah. 
And Allah knows best.