This is a wonderful question, and it's something I struggle with as well. I'm really struggling with it at the moment, because the idea I'm currently composting is about a girl who's determined to always live in the moment and not worry about the future. So ... her goal is basically to not have goals. When I realized that, I kinda stared at the page, thinking, "Uh...."
Sometimes what happens for me is I try to make the external goal too complicated. I've
found that my character's internal goal is often something they don't realize they want and the external goal is something they do know they want. Not always, of course, but frequently.
Susan May Warren does a wonderful job with external goals. In her book Happily Ever After, the main character is opening up a book store. Her external goal is to get the building ready to open by the start of summer, when tourists flock to their town.
Often, external and internal goals work together. In So Over It, Skylar's external goal is to figure out what really happened on the night she was nearly date-raped. Her internal goal is to forgive herself for bad choices she once made.
So start by looking at your story and determining what it is your character knows he or she is working toward. If your character doesn't have a goal they know about, find one for them.
It can work well to have the external and internal goals work together, like in my example of So Over It, but it can also work to have the external and internal goals battling each other. Like to have a character whose internal goal is to live a simpler life ... but whose external goal is to run a boutique gift shop. Boutique gift shops sell things luxury items, so that would really battle with the character's desire to live simpler.
I hope this helps a little. Take comfort in the fact that you're not alone!