As you now know, trim ( handles, etc) for faucets and showers are not generic at all, but rather are specific parts for a particular brand and model. They cannot be "mix and match". Your first alternative is to identify the present brand and model, then go to that website, or to a plumbing supplier, and see what trims are currently available. Based on the description of what you ordered, you will NOT find anything like that to retrofit to an old shower. At best, you may be able to change over to a newer brush nickle finish, or if it is a very common brand, you might even find a venetian bronze.
The best...not easiest....alternative, is to open the wall up, preferably from the back side, and install a new valve set. Most of us would recommend against that set you bought if it does not have a big name on it. If it is not Kohler, Moen, Delta, Price Pfister, Symmons......then you will have serious difficulty down the line getting repair parts.