Fate in Islam
Question

Is our fate completely predestined? Some say we have a choice of which path to choose but what you will find at the end of the path has been chosen for you by Allah. I have also read that qadar was perhaps created by Jahm b. Safwan, not Allah.
Where can I find the information in the Quran and if there is a set destiny how much of it is chosen by God? Is there really a set day on which I will have birth and another for earthly death, for whom I will marry?
What if I did meet the man I am supposed to marry but I somehow choose the wrong path (so I may believe) only to realize I did wrong. Will he come in my path again or have I altered my destiny so that my punishment may be that I can not have this person in my life?
Praise be to Allah.Belief in al-qadar
Belief in al-qadar (the divine will and decree ) is one of the pillars of faith. The Prophet (peace and blessings of Allah be upon him) said, when he answered Jibril’s question about faith: “(It means) believing in Allah, His angels, His Books, His Messengers and the Last Day, and to believe in al-qadar (the divine decree) both good and bad.” 
What is al-qadar?
What is meant by al-qadar is that Allah has decreed all things from eternity and knows that they will happen at times that are known to Him, and in specific ways, and that He has written that and willed it, and they happen according to what He has decreed. [al-Qada wa’l-Qadar by Dr ‘Abd al-Rahman al-Mahmoud, p. 39] 
Basics of belief in al-Qadar
Belief in al-qadar is based on four things: 

Knowledge, i.e., that Allah knows what His creation will do, by virtue of His eternal knowledge. 
Writing, i.e., that Allah has written the destiny of all creatures in al-Lawh al-Mahfuz. 
Will, i.e., that what Allah wills happens and what He does not will does not happen. There is no movement in the heavens or on earth but it happens by His will. 
Creation and formation, i.e., that Allah is the Creator of all things, including the actions of His slaves. They do their actions in a real sense, and He is the Creator of them and of their actions. 

Whoever believes in these four believes in al-qadar. 
Reference to Al-Qadar in the Quran
The Quran affirms these things in numerous verses, such as the verses in which He says (interpretation of the meaning): 

“And with Him are the keys of the Ghayb (all that is hidden), none knows them but He. And He knows whatever there is in the land and in the sea; not a leaf falls, but He knows it. There is not a grain in the darkness of the earth nor anything fresh or dry, but is written in a Clear Record” [al-An'am 6:59]
“No calamity befalls on the earth or in yourselves but it is inscribed in the Book of Decrees (Al-Lawh Al-Mahfuz) before We bring it into existence. Verily, that is easy for Allah” [al-Hadid 57:22] 
“And you cannot will unless (it be) that Allah wills, the Lord of the ‘Alamin (mankind, jinn and all that exists)” [al-Takwir 81:29] 
“Verily, We have created all things with Qadar (Divine Preordainment of all things before their creation as written in the Book of Decrees Al-Lawh Al-Mahfuz)” [al-Qamar 54:49]

Muslim (2653) narrated that ‘Abd-Allah ibn ‘Amr ibn al-‘As said: I heard the Messenger of Allah (peace and blessings of Allah be upon him) say: “Allah wrote down the decrees of creation fifty thousand years before He created the heavens and the earth.” He said: “And His Throne was upon the water.”  
Was al-qadar invented by al-Jahm ibn Safwan? 
Hence it should be clear to you that the view that al-qadar was invented by al-Jahm ibn Safwan is a view that has no sound basis. Al-qadar was not created, rather creation comes under the heading of belief in al-qadar. 
Al-Jahm went to extremes in affirming the divine will and decree, and claimed that people are compelled to do what they do and have no choice. This is a false view. 
Belief of Ahl al-Sunnah wa'l-Jamaa'ah about al-Qadar
The belief of Ahl al-Sunnah wa'l-Jama'ah is that a person has freedom of will , and hence he will be rewarded or punished. But his will is subject to the will of Allah, and nothing can take place in the universe that is not willed by Allah. 
What some people say, that we have the choice to follow whatever path we want but at the end of this path you will find what Allah has decreed for you, is a correct view. Allah says (interpretation of the meaning): 
“Verily, We showed him the way, whether he be grateful or ungrateful” [al-Insan 76:3] 
“And shown him the two ways (good and evil)?” [al-Balad 90:10] 
“And say: “The truth is from your Lord.” Then whosoever wills, let him believe; and whosoever wills, let him disbelieve” [al-Kahf 18:29]
View of Ahl al-Sunnah with regard to man’s deeds
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said, explaining the view of Ahl al-Sunnah with regard to man’s deeds:  
“People act in a real sense, and Allah is the Creator of their actions. A person may be a believer or a kafir, righteous or immoral, he may pray and fast. People have control over their actions, and they have their own will, and Allah is the Creator of their control and will, as Allah says (interpretation of the meaning): 
“To whomsoever among you who wills to walk straight. And you cannot will unless (it be) that Allah wills, the Lord of the ‘Alamin (mankind, jinn and all that exists)” [al-Takwir 81:28-29]” (al-Wasitiyyah ma’a Sharh Harras, p. 65)
Is your marriage predestined? 
Marriage is one of the things that Allah has decreed. The person whom you will marry is known to Allah: He knows who he is, when he was born, where and when he will die, how he will be towards you, and other details. All of that is known to Allah and He has written it in al-Lawh al-Mahfuz (the Book of Decrees), and it will inevitably come to pass as Allah has decreed. 
If Allah has decreed that you will marry one person, but you choose someone else, then no matter how long it takes, you will marry that person. But your marriage to someone else is also decreed, because there is nothing that is not decreed by Allah. It may be decreed for a woman to marry So and So the son of So and So, and he comes to propose marriage but she refuses him, and marries someone else, then he (the second man) dies or divorces her, then she accepts the first one. All of that is decreed, and it is decreed for her to marry So and So the son of So and So after initially refusing him and after some experience or trials etc. 
It may be decreed for a woman that a righteous man will propose marriage to her, but she will refuse him and he will never come back to her, and she will marry and live with someone else who is more or less righteous, according to what Allah has decreed. 
Because man does not know what is decreed for him, what he must do is to adhere to Shari`ah and abide by its commands and prohibitions, and to seek the help of Allah and pray to Him for guidance (istikharah ) concerning all his affairs, whilst implementing the means, one of the most important of which is consulting sincere people who have relevant experience. 
If a righteous man proposes marriage to a woman, she should pray to Allah for guidance (istikharah) and agree to marry him. If things then go smoothly, this is an indication that what is good for her is to marry him.  
In conclusion, man should study the Shari`ah of Allah and follow the commands of Allah even if he feels reluctant, and avoid what Allah has forbidden even if he is attached to it. All goodness is to be found in obeying Shari`ah. Allah says (interpretation of the meaning): 
“Jihad(fighting in Allah’s Cause) is ordained for you (Muslims) though you dislike it, and it may be that you dislike a thing which is good for you and that you like a thing which is bad for you. Allah knows but you do not know” [al-Baqarah 2:216]
He should not look at al-qadar as an excuse to ignore commands and do forbidden things, rather he should look at it in a way that will make him content with whatever painful experiences happen to him. 
And Allah knows best.