Allaah The Almighty Says (what means): {Whoever Allaah sends astray— there is no guide for him.} He also Says (what means): {And whoever Allah guides— for him there is no misleader.}
Therefore, even if we observe the Sharee‘ah (Islamic law) we will not be guided unless Allaah The Almighty wants this, therefore why are we held accountable for something that is beyond our will?
Since human abilities differ, why are their obligations the same? Since hearts are between two fingers of the fingers of Allaah The Almighty and since He changes hearts the way He wants— as narrated by ‘Abd Allaah ibn ‘Amr, may Allaah be pleased with him, in Saheeh Muslim — then everything is beyond our control. For example, I might be an observant Muslim, yet Allaah The Almighty may change my heart and I would become a bad person with no input from me.

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.

Muslims must believe in the following facts:

1-      Allaah The Almighty does what He wills. He Says (what means): {He is not questioned about what He does, but they will be questioned.} [Quran 21:23]

2-      Allaah The Almighty is Just and He never wrongs anyone. Allaah The Almighty Says (what means):

•        {And your Lord (Allaah) is not ever unjust to [His] servants.} [Quran 41:46]

•        {Indeed, Allaah does not do injustice, [even] as much as an atom's weight.} [Quran 4:40]

3-      Allaah The Almighty guides whoever He wants to the right path, and makes whoever He wants go astray. Allaah The Almighty Says (what means):

•        {Whoever Allaah guides— he is the [rightly] guided; and whoever He sends astray— it is those who are the losers.} [Quran 7:178]

•        {And whoever Allaah leaves astray— there is not for him any guide.} [Quran 7:168]

4-      Allaah The Almighty holds a person accountable for their actions. Allaah The Almighty gave man reason, hearing, perception and will so that people can distinguish between good and evil and right and wrong. Allaah The Almighty Says (what means):

•        {For whoever wills among you to take a right course.} [Quran 81:28]

•        {He has succeeded who purifies it, and he has failed who instills it [with corruption].} [Quran 91:9-10]

•        {Indeed, We guided him to the way, be he grateful or be he ungrateful.} [Quran 76:3]

As human beings have the option to choose to do or not do something, they are expected to fulfill their Sharee‘ah (Islamic law) obligations and so they are rewarded when they do good deeds and punished when they do bad deeds.

You indicated that humans are held accountable for something that is beyond their will but this is obviously wrong. Every sensible person knows that he or she has freedom of choice and does good and bad deeds voluntarily. The fact that Allaah The Almighty knows in advance the destiny of a person does not mean that this person is compelled to follow a certain path. Allaah The Almighty Says (what means): {So whoever wills—- let him believe; and whoever wills — let him disbelieve.} [Quran 18:29]   

You asked why every human must fulfill the same obligations although human capabilities differ. The Sharee‘ah does not treat all people as equal in terms of duties and obligations, rather there are exceptions under the Sharee‘ah. Those who are unable to fulfill an obligation have relevant rulings that suit them, and those who are able to fulfill an obligation have relevant rulings that suit them. Examples of such exceptional Sharee‘ah rulings include Tayammum (dry ablution), passing wet hands over Khuffs (leather socks), performing two-Rak‘ah Salah (prayer) instead of four, performing Salah while sitting or lying, the concession of not observing fasting) and doing Hajj (pilgrimage) on behalf of others.

As for determination, everyone should develop their own determination and will. Allaah The Almighty helps those who want to develop their determination, especially those who take the necessary steps towards seeking guidance. Allaah The Almighty Says (what means):

•        {And Allaah increases those who were guided, in guidance.} [Quran 19:76]

•        {And those who strive for Us— We will surely guide them to Our ways.} [Quran 29:69] 

You said that since hearts are between two fingers of the fingers of Allaah The Almighty and since He changes hearts the way he wants, everything is beyond our control. In fact you are ordered to do obligatory deeds and to remember that Allaah The Almighty gave you freedom of choice. When the Sahaabah (Companions) asked the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, whether they can stop doing the obligatory deeds, given that Allaah the Almighty decreed everything in advance, he commanded them to keep on doing good deeds since everyone is guided to do the deeds that will lead him to the fate decreed for him. [Al-Bukhari and Muslim]

It is a wonder why people do not just concern themselves with doing the deeds required of them, instead of occupying themselves with discussing unknown matters, given that they are not ordered to discuss them.

You said that you might be an observant Muslim, yet Allaah The Almighty may change your heart so you become a bad person without committing any sin. Allaah The Almighty does not deprive anyone of the reward of their good deeds. In fact, He never wrongs anyone. He loves those who do good deeds and treats his slaves according to what they think of Him. Indeed He is more merciful to His slave than a mother is to her child.

Those who do not do the obligatory deeds will have a bad end; likewise, those who do them to impress others will also have a bad end.

Therefore, you must think well of Allaah The Almighty. You must know that Allaah The Almighty never wrongs anyone and that He is the Ever-Forgiving, the Ever-Merciful and the Ever-Honorable. You must also know that Allaah The Almighty gives His slaves many bounties, even though some of them are careless, disobedient or even disbelievers.

Occupy yourself with doing what benefits you and strive to do good deeds to be one of the winners. Indeed today you can do good deeds and you will not be questioned, but in the Hereafter you will be questioned and not able to do such things. You should therefore imitate the Sahaabah (Companions) who strove to do good deeds as much as they could, although they knew that everything was decreed in advance by Allaah The Almighty.

Allaah Knows best.