Asking such a question almost seems a bit insane. While I agree that there certainly seems to exist an objective external reality of which I seem to be a part of, I can never experience existence in a truly objective way (by not being a part of it and just looking in). The way things really appear to me is that everything is ultimately subjective to me.
There are things that I can immediately control in my subjective experience and things that are out of my control, but that are nevertheless part of me because ultimately all perception and understanding takes place within my mind. Since I am unable to take a truly objective view to assess my own reality, because I can’t step out of everything in order to look back in, a few strange and strongly solipsistic interpretations of reality seem just as plausible to me as more accepted objective ones.
For example: what if all existence appears to take place exclusively from my point of view because I am the only (or first) conscious entity in the world? What if every other human is just a human behavior machine (a philisophical zombie) and I am the sole conscious observer of the world? I would be in a sense a prototype or at least the current (unique) means for the universe to be self aware.
Or considering simulation theory but with a solipsistic twist: what if I reside in a simulation of my own making? The whole universe with everything in it is my own creation in order to simulate this being called me. In order to simulate the current me to the highest fidelity possible, I have to naturally follow through to the fullest extent which includes adhering to the rule that even though everything is ultimately subjective to and created by me, I can only control (and even understand) just a subset of my experince; the rest appears out of my control in order to simulate this thing I call objective (or external) reality. If the current state of affairs weren't as they are, then I would be simply running a different simulation than the current one I am in. It actually would not be neccesarily distinguishable to me if I am conscious prototype of the universe not of my own making or if I am playing out my own simulation of the universe.
The fact, however, remains that all of existence appears to take place exclusively through my point of view to me. Is there a way that I can prove these extreme positions false to myself?