I have been listening and reading related to this. But never got a clear understanding about this. Is the person I'm going to marry already chosen by Allah? Is it that no matter what happens I'm destined for that one person? I have been hearing people saying if its written it will happen dont stress too much. Please answer my question.
Everything is destined for the creation. Everything in this universe is predestined. How we, the creations, perceive it is in the form of free will. However, Allah Subhanahu wa ta'ala has already outlined everything.
From Ayah Al kursi:
"Ya'lamu ma baina aidihim wa ma khalfahum, wa la yuhituna bi shai'im-min 'ilmihi illa bima sha'a" He knoweth What (appeareth to His creatures As) Before or After or Behind them. Nor shall they compass Aught of his knowledge Except as He willeth
As for your stressing over your situation, that is also predestined; as we can see from the Ayah above. Whether you end up with your partner, or you don't. That is a predestination that Allah Subhanahu wa ta'ala has already prescribed for you.
You can perform istikharah.
narrated by Jabir ibn ‘Abd-Allah al-Salami, who said:
"The Messenger of Allah (peace and blessings of Allah be upon him) used to teach his companions to make Istikharah in all things, just as he used to teach them Surahs from the Qur'an. He (SAW) said: 'If any one of you is concerned about a decision he has to make, (or in the version narrated by Ibn Mas'ood as: 'if any one of you wants to do something...) then let him pray two rak'ahs of non-obligatory prayer and say (after the Salah)