Praise be to Allah.
Belief in the divine will and decree is one of the pillars of faith, and the Muslim’s faith is not complete unless he understands that whatever befalls him could not have missed him, and whatever missed him could not have befallen him. The believer has no choice but to bear with patience whatever Allah, may He be exalted, decrees for him of calamities. That is a sign of his having perfect faith. Whoever bears that with patience, Allah, may He be exalted, will grant him his reward in full and without measure on the Day of Resurrection.
You should not think that what Allah, may He be exalted, has decreed for you is entirely bad, for there is nothing in the actions of Allah, may He be exalted, that is of that nature. There is great wisdom in what Allah, may He be exalted, decrees for His slaves, and what you dislike of the situation that you are in now is something in which there may be a great deal of good of which you are not aware. Allah, may He be exalted, says (interpretation of the meaning): “perhaps you dislike a thing and Allah makes therein much good” [an-Nisa’ 4:19]. Al-Bukhaari (5649) narrated that Abu Hurayrah (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “When Allah wills good for someone, He tries him with calamities” – so that He may reward him for that.
The fact that Allah is testing you with that disability does not mean that Allah, may He be exalted, does not love you; rather perhaps the opposite may be true. It was narrated from Anas ibn Maalik (may Allah be pleased with him) that the Prophet (blessings and peace of Allah be upon him) said: “The greatest reward comes with the greatest trial. When Allah loves a people He tests them. Whoever accepts that attains His pleasure but whoever is discontent with that earns His wrath.” Narrated and classed as hasan by at-Tirmidhi (2396); also narrated by Ibn Maajah (4031).
One of the greatest benefits for the one who bears a trial with patience and seeks reward with Allah for that is that he could meet his Lord with no sins left. It was narrated that Abu Hurayrah (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “Calamity may continue to befall believing men and women with regard to their own well-being, their children and their wealth, until a believer meets Allah with no sin on him.” Narrated and classed as saheeh by at-Tirmidhi (2399).
Hence those who bore trials with patience and sought reward with Allah will have the greatest status on the Day of Resurrection, to the extent that those who enjoyed well-being in this world will wish to be like them. It was narrated that Jaabir (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “On the Day of Resurrection, when those who faced trials are given their rewards, those who enjoyed well-being will wish that their skins had been cut to pieces with scissors in this world.”
Narrated by at-Tirmidhi (2402); classed as hasan by al-Albaani in Saheeh at-Tirmidhi.
Hopefully, some trustworthy Muslims will take it upon themselves to arrange marriages for sisters who are affected by physical disability, for such a great deed will bring an immense reward, in sha Allah.
The Muslim to whom Allah has granted physical well-being should not look down on or scorn those who are affected by disability. Let him praise Allah, may He be exalted, for having spared him from that with which He has tested others, but it is not appropriate for him to let [the person with a disability] hear him say that du‘aa’, lest it hurt his feelings. Whoever is grateful for the blessing of well-being should offer whatever service and care he can to those who are affected by disabilities.
It is very important to read the answer to question no. 71236, which explains the believers’ attitude towards tests and trials.
With regard to your thinking that you will only have half the reward because you pray sitting, that is not correct. Rather you will have the reward in full, in sha Allah. The half-reward is only for the one who offers a naafil (supererogatory) prayer sitting when he is able to stand. As for the one who prays sitting because he is excused due to illness, he will have the reward in full.
An-Nawawi (may Allah have mercy on him) said:
The ummah is unanimously agreed that the one who is not able to stand in an obligatory prayer may pray it sitting, and he does not have to repeat it.
Our companions said: Nothing is detracted from the reward he would have if he were standing, because he has an excuse. It is proven in Saheeh al-Bukhaari that the Messenger of Allah (blessings and peace of Allah be upon him) said: “If a person becomes ill or travels, there will be recorded for him a reward like that of what he used to do when he was not travelling and was healthy.”
Thirdly: With regard to the issue of thinking of sin, as mentioned in your question, we have discussed it previously in detail in the answer to the question no 171726 .
And Allah knows best.