Hi patmurphy and welcome to the site.
Aww, bless you, in all honesty, you're not doing anything wrong, and it will curdle. When you cream butter and sugar and then start adding eggs, you have a lot of fat and water content going at the same time and the more eggs you add, the more it will split. If you add a spoonful of the flour you are using after each egg it will help bind the mixture together more.
The same thing happens when I make a fruitcake, and there are lots of eggs needed for that. The wet ingredients can sometimes resemble a bowl of sick for want of a better phrase, but once you add the dry ingredients, it will start coming together.
View attachment 5505
This is a close-up before any dry ingredients get added. As you can see, it looks awful, but once you start adding lots of dry ingredients to it, it all combines together and works beautifully.
I have put a link to my fruitcake tutorial below, if you scroll down the pictures you'll see the curdling.
As we're entering that time of year again and with Stir Up Sunday just around the corner. I thought I would put together a mini-tutorial with a small video showing you how to make your own Christmas Cakes. Tip: Take your butter out of the fridge 30 minutes before starting your cakes to allow...
www.helpmebake.com
I also mention curdling in my baking phrase post too.
What is a baking phrase? A baking phrase is a word or prompt that will subtly sit in your recipe, guide or tutorial. It is a small instruction that is placed to help you achieve the best bakes. Some will be familiar to you, some will be self-explanatory and some of them you may not be too sure...
www.helpmebake.com
I hope this helps.
Best wishes.
Angie