Importing An Endnote Database into An Already Existing Database

I have created a large endnote database but now need to import it into another database that is already established and am having trouble with accomplishing this.

How are you trying it?  It is pretty straight forward.

With the library open that you want to import INTO, you should File>import and set Import Option> EndNote Library, Duplicates set as you prefer (I usually import duplicates into a separate library, so I can quickly retrieve one, if it is better than the existing one) and  I usually set  Text Translation to Unicode (UTF-8).  If you would prefer to keep the newer library copies of any duplicates, then first open the smaller library, but then it will take longer.  

If there aren’t any duplicates, you can also open both libraries, and select all from one and drag it into the other.

You need to tell us what is your “existing database” software, and whether it has tab-delimited or comma-delimited text import functions. Some software can import tagged texts, but how to set up these import functions is not the question here. Endnote has export functions, like tab delimited or tagged texts, but it is all dependent on your “existing database” software.