Is One’s Spouse Written in Islam?
Question

What does Islam say about life partner? Is she/he fixed by Allah or Allah has left it for us to decide?
Praise be to Allah.First of all, we should realize that there is no contradiction between our belief that Allah has decreed everything, and our belief that He has given us free will which enables us to do things. Confirming the free will of His slaves, Allah says (interpretation of the meaning): "To whomsoever among you wills to walk straight" [At-Takwir 81:28]

Our will operates within the will of Allah , not outside of it, as Allah says (interpretation of the meaning): "But you cannot will, unless Allah wills…" [Al-Insan 76:30] 
Thus it is not permitted to try to cancel one idea out with the other, or to deny either of them. Allah has confirmed both, that man has the power to choose, and that nothing can operate outside of His Will, may He be Glorified.

If we apply this to the question asked, we will see that man has freedom of will through which he can choose whichever woman he wishes to marry , and that whatever choice he makes has already been decreed by Allah. 
The man’s choice is the means by which he gets what he wants, but some obstacles may come between the person and what he wants, in which case he will realize that Allah has not decreed this for him, for some reason which is known to Him.
Everything that Allah does is good, and His slaves do not have knowledge of the Unseen or of how things will end. A person may regret missing out on something when it is good that he did not have it, or he may hate something that happens to him when it is good that it happened to him, as Allah tells us (interpretation of the meaning): "… 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]

May Allah bless our Prophet Muhammad.