People who think they're lousy at planning are usually wrong.
Planning is rarely an issue for anyone and, I'd argue, that everyone has a plan in mind or on paper.
Execution, on the other hand, is a real problem for people.
Turning a plan into action seems to stump the best of us.
So let's place the emphasis where it belongs. Be honest.
Instead of saying, "I'm not great at planning" say, "I'm not great at executing a plan."
At least then you can get the help you need.