Praise be to Allah.
The word of sincerity (ikhlaas) and the testimony of Tawheed is the foundation of faith, the sign of salvation, and the proof of success; humankind and the jinn were not created except to believe in it properly and act accordingly.
It includes all three types of Tawheed: Tawheed ar-ruboobiyyah (oneness of divine Lordship), Tawheed al-uloohiyyah (oneness of divinity, which is oneness of worship), and Tawheed al-asma’ wa’s-sifaat (oneness of the divine names and attributes).
The one who utters this testimony must believe in that and bring it to mind when uttering these words; he must affirm it completely in his heart, without any confusion or any element of shirk, and without any sense of objection or resentment.
The soul’s need to know and affirm Allah’s oneness is greater than its need for food, drink and air. It is an innate and instinctive need. What is required of the accountable person is to strive to learn about it and acquire knowledge about it that will enable him to fulfil its conditions, and will help him to rid himself of anything that could undermine his sincerity and faith when saying it.
The three types of Tawheed are interconnected, so the one who affirms one of them must affirm all of them. Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him) said:
Affirming the oneness of divine Lordship inevitably leads to affirming the oneness of divinity; it points to it and necessitates it. Affirming the oneness of the divine names and attributes is necessitated by affirming the oneness of divine Lordship, because the One Who is the Creator, the Provider and Sovereign of all things is the One Who deserves to have all the most beautiful names and sublime attributes. He is the One Who is perfect in His Essence, His names, His attributes and His actions. He has no partner and nothing resembles Him; vision cannot perceive Him, and He is the All-Hearing, All-Seeing.
The one who properly understands all three types of Tawheed, keeps them in mind and adheres to their meanings will realise that Allah is indeed the One, and He alone is the One Who is deserving of worship, to the exclusion of His creation. Whoever omits one of these three types of Tawheed will omit all of them, because they are interconnected, and a person’s Islam cannot be sound except with all three." (Majmoo‘ Fataawa Ibn Baaz 1/38-39).
So long as you believe in all of that, with certainty, there will be no shortcoming or problem with your testimony and there will be nothing wrong with your repentance, such that you will need to renew it. A person may not bring to mind some of what he knows in some situations, and in some situations he will have in mind one of the names or attributes of Allah, may He be exalted, without that meaning that there is a shortcoming in his belief in the other divine names and attributes.
By the same token, he may do an act of worship and obedience to Allah, and what is foremost in his mind is the concept of worshipping Allah alone and being sincere to Allah in his actions.
He may be seeking provision or relief from harm, so what is foremost in his mind is the belief that Allah is the only Lord and Controller of His creation, the only One Who is in control and disposes of people’s affairs, and so on.
And Allah knows best.