I used to make these mistakes in my prayers in the past:
1- Failing to make the intention in a correct manner; I used to pronounce it loudly, like the Azaan.
2- I did not say As-Salah Al-Ibraheemiyyah (the wording of sending peace and blessings of Allaah on the Messenger, sallallaahu ‘alayhi wa sallam, after the final Tashahhud).
3- I did not make Qunoot in the morning prayer.
Please advise me regarding my inquiries. Do I have to redo my prayers? If I must, how can I do this, given that I prayed in such a manner for five years?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

Intention is the determination to do something and it is made by the heart.  The scholars disagreed as to whether pronouncing the intention with your tongue is a Sunnah act or not. The correct opinion is that it is not a Sunnah act, but if the person pronounces it, even audibly, his prayer is valid and all praise be to Allaah Almighty.

As for sending peace and blessings upon the Prophet, sallallaahu ‘alayhi wa sallam, in the final Tashahhud of the prayer, the scholars hold different opinions regarding its ruling:

According to the opinion adopted by Ash-Shaafi‘i, may Allaah  have mercy upon him, and his followers, which is also the opinion of the Hanbali scholars as stated in Kashshaf Al-Qina‘ and Matalib Uli An-Nuha: sending peace and blessings upon the Prophet, sallallaahu ‘alayhi wa sallam, is obligatory in the final Tashahhud, and the prayer of whoever leaves it is invalid. They support their view with the Hadeeth narrated on the authority of Ka‘b ibn ‘Ujrah as he said:

I asked the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, "O Messenger of Allaah, how should we invoke peace and blessings upon you in our prayer?" He, sallallaahu ‘alayhi wa sallam, said: "Say: 'Allaahumma salli ‘ala Muhammmadin An-Nabiyy Al-Ummiyy wa ‘ala ali Muhammadin kama sallayta ‘ala Ibraheema wa ‘ala ali Ibraheema wa barik ‘ala Muhammmadin An-Nabiyy Al-Ummiyy wa ‘ala ali Muhammadin kama barakta ‘ala Ibraheema wa ‘ala ali Ibraaheema innaka Hameedun Majeed.' (O Allaah, confer your peace upon Muhammad, the illiterate Prophet, and his family as You conferred peace upon Ibraheem and his family. O Allaah, confer your blessings upon Muhammad, the illiterate Prophet, and upon his family as you conferred Your blessings upon Ibraheem and his family. Indeed, You are Praiseworthy and Glorious)."

Imaam An-Nawawi said:

"This Hadeeth is narrated by Abu Hatim, Ibn Hibbaan in his Saheeh, and Abu ‘Abdullaah Al-Hakim in his Saheeh. It is also narrated by Ad-Daraqutni and Al-Bayhaqi, and they use it as evidence. Ad-Daraqutni said: "This has a good Isnad (chain of narrators)," while Al-Hakim said: 'This is an authentic Hadeeth.'"

The Hadeeth is also narrated in Saheeh Al-Bukhari, Saheeh Muslim, and other books; but we chose this narration because it includes the companion's saying: "How should we invoke peace and blessings upon you in our prayer?" which is the point of the question. It should be known that the minimum obligation, according to the scholars who make it obligatory, is to say: "O Allaah, confer your peace upon Muhammad.” Whoever says so has fulfilled the obligation, his prayer is valid, and he is not required to say the full wording of As-Salah Al-Ibraheemiyyah.

On the other hand, the Hanafi and Maaliki scholars view that sending peace and blessings upon the Prophet, sallallaahu ‘alayhi wa sallam, is recommended, but not obligatory. Hence, we say to the questioner: Do not worry because your prayers are valid according to these two schools of Fiqh. However, we advise you to abide by invoking peace and blessings upon the Prophet, sallallaahu ‘alayhi wa sallam, in your prayers.

Concerning making Qunoot in the morning prayer, the scholars have different opinions regarding its ruling too. The Shaafi‘i and Maliki scholars view that it is recommended. Whereas Ahmad and Abu Haneefah say that it is not an act of the Sunnah.

In order for the Qunoot and other invocations of prayer to be valid, it should be said to yourself silently, and this is based on the Shaafi‘i and Hanbali schools of Fiqh. Other scholars mention that one should move his tongue and lips with their words. Hence, if a person says the supplication of Qunoot in such a way that he cannot hear himself, then his Qunoot and prayer are valid, and he is not required to redo it. Similarly, the prayer of one who does not make Qunoot at all is valid as well.

Allaah Knows best.