My results are different from my original poster title now. How I should prepare the poster? I am a graduate student in an interdisciplinary psychological field. With the help of a paper I have read today, I have realized that a result of mine is now invalid: I missed a covariate in my analysis and when I include it, my result is non-significant. Fortunately it was not that much about the main point of my conclusion, but the non-significant term happens to be in my abstract title (and my abstract is printed on the abstract book of the conference now!).

I told this to my advisor and he said: “you cannot control every single thing, it is not that important”. But I have controlled it and it did affect my results, and I actually had that particular covariate in my analysis while I was writing the abstract and my advisor told me to exclude it.

I do not know what to do now. I cannot pretend like the result is still there; that conference is very important in my field and I do not want to regret it later. I do not think that it is ethical. I cannot pretend like I did not know that particular covariate is actually important for my analysis, because I have read the paper! I do not know what to do if someone asks me: “why you did not include this covariate?” I do not know whether it is okay to change the title and exclude some results from the poster, even when the abstract (in the abstract book) says another thing.

What should I do? I am very confused and hopeless.
Of course you shouldn't pretend that the result is still significant, when you know now that it isn't. As you have noted, that would certainly be unethical.

Prepare your poster using the results that you believe are correct now. If anyone asks about the difference between the poster and the abstract, just explain what happened. (You aren't the first to make a mistake and catch it later, nor will you be the last.)  You can also email the conference organizers to find out if they can update the poster title and abstract (online at least).

Occasional mistakes are common in science, as in everything else; don't stress about it too much, just learn from it for the future. It's great that you caught it now, before your poster presentation, so that you can correct it.