So I want to make my dll only be injectable by my injector and I figured that a good way to do that is by only letting my dll be able to open when the injector is running. But I have no Idea how to do that.
As Mayur mentioned in comments, your DLL can enumerate running processes using EnumProcesses(), and get their filenames looking for a match to your injector. You can do that in your DLL's entry point function during the DLL_PROCESS_ATTACH stage. If the injector is not detected, your entry point can return FALSE to abort the load.
Alternatively, a much simpler way to detect your injector is to have it create a named mutex via CreateMutex() in the global kernel namespace, and then have the DLL entry point try to access that same object via OpenMutex(), and fail to load if the mutex does not exist. See Using Named Objects for more details.