I don't believe OP or anyone else is claiming in this thread that ghosts are a fact
The OP's title is "Pretty sure that I have a ghost!"
In order to "have" a ghost, ghosts would have to exist in the first place, therefore the statement argues the existence of ghosts, and is therefore positing ghosts as fact.
However, if someone is making an assertion to the contrary, that should also have some evidence
Err no.
Once again, the claim being made is that ghosts exist. There is no need for anyone to provide any sort of proof to the contrary, because the onus lies with those making the factual claim, not those questioning the lack of any evidence whatsoever to support the claim.
It's all subjective beliefs at this stage
It's really not. "Subjective" refers to matters of opinion or taste, matters which don't depend on fact, and therefore can never be either true or false.
Objectively, there is nothing at all which suggests the existence of ghosts, so it's entirely an objective, not subjective view, to conclude that they do not exist.
There was a time when those claiming the existence of micro-organisms like bacteria causing diseases were laughed at, without presenting any evidence to back their dismissal
Indeed they were laughed at, from a position of ignorance as that turns out. However, in short order those making the claim that microorganisms existed were able to prove their hypothesis correct and entirely validate their theories. Human cultures have been proclaiming the existence of ghosts for thousands of years, and yet, despite being advanced to the point whereby we can detect things invisible to the naked eye, detect things outside of our own Galaxy even, we are still apparently incapable of quantifying something which has been speculated about since humans first came up with the concept eons ago, something which supposedly appears right in front of human beings, so is supposedly detectable with the human eyeball.
without presenting any evidence to back their dismissal
That will be, again, because the onus to provide evidence does not lie with sceptics.
and their rational was similar to yours
It wasn't really, because at the time when microorganisms were unknown, there were still people proposing perfectly viable hypothesis purporting their existence. The technology required to prove their existence did not exist, hence the scepticism, but that technology soon followed, spurred on by the seeming plausibility of the hypothesis.
I'm still waiting on any sort of remotely credible hypothesis for the existence of ghosts.