Are the death obsessions that have afflicted me, and what I saw in my dream, a reality or just obsessions, and what should I do about it?

Abandon your obsessive thoughts, for no soul shall die until it has fulfilled its sustenance and its term. And when the term comes, it will neither be delayed nor advanced. Allah (the Most High) says: "And never will Allah delay a soul when its time has come" [Al-Munafiqun: 11], and He says: "And for every nation is a [specified] term. So when their time comes, they will not remain thereafter for an hour, nor will they precede [it]" [Al-A'raf: 34].

Let your fear of death be a motivation to prepare for it through righteous deeds, for death is a comfort for the believer. Do not think about when and how it will come, but strive in obedience to Allah. If the obsessive thoughts persist, consult a psychiatrist.