According to Islam, has my future husband already been fixed from the day I born? Or will it change according to my decisions?
Allah has written the fate of everything. It will not change. Whatever decision you make is written there.
Islam doesn't restrict anyone from gaining knowledge, and doesn't have any law about marrying age. You have to choose a good husband for yourself, whether now or 5 years later. You cannot blame you parents if you choose a wrong man or you didn't voice your opinion about the groom when they get you one. Hadith here. So it doesn't matter when you marry, it will not change anything in fate. And it is not true that you will get bad husband if you marry lately.
Studying will only make you more knowledgeable and mature. It is totally up to you when you want to marry. But wherever you go, always follow Islam's law, so that your parents won't say "See this is why we didn't want you to study".
A woman in Islam can study and increase her cultural and intellectual level without a problem, if Fitna is avoided, if you feel that the place where you study doesn't present a danger on you and your religion, no guys approach you and you are not fascinated or have bad thoughts, then you can continue studying.
You also shouldn't confuse and mix studying with not getting married, and getting married with stopping your studies, you can propose to your husband that you want to study and finish your 3 years plan and given the right arguments the right guy for you will accept such condition without a problem if you feel that you can give your man and you home their rights.
As long as you obey Allah and you make the good decisions don't fear your faith, it will come to you in its best form and you'll be pleased, just make sure God and Islam's teachings always come first.
Be sure to choose the religious muslim man, and think of your priorities on the long term.
May Allah guide you.