I agree that if it's really floor length, it would be better if you shortened it, but I think that maybe you wouldn't have to, if you put something underneath it and wore it like a jumper instead of a stand alone dress. A pretty blouse in a spring print, for example, would soften the "bridesmaid" vibe, I think. As would a sweater or shrug over the top, in a slightly less formal style than the dress.
As others have said, shoes and jewelry would make a big difference also. To a wedding, I wouldn't wear a simple pearl choker, for example. Because that, at least to me, would give a stronger "bridesmaid" vibe. And definitely no shoes dyed to match (if you still have them, or ever had them). But to another type of formal event, that would probably be fine.
I think what I would worry about most, now that I think about it, is not that it looks like a bridesmaid dress as such. My concern would be, how formal is the wedding? If it's in somebody's backyard, then probably this dress is too formal even if shortened. A black tie, or even white tie, evening affair in which the wedding party has floor length dresses also? This dress would fit right in for a guest, I would think.
ETA: I am basing my advice somewhat on the fact that "maxi dresses" (that literally touch the floor) seem to be making a comeback around here, even in very informal fabrics and styles. YMMV.