Why do you think it was easier for Martha to confess her feelings to John Smith than it was the Doctor since technically they're the same person?

I think it was because she thought he was so human that he'd forget everything she said to him when he turned back. Also, she said she loved the Doctor to make John Smith think The Doctor wasn't all that lonely, as he was loved.

I think that she thought he wouldn't remember or she was trying to get her feelings across to someone who might remember what she said so John Smith was her best chance as it was still technically the Doctor.