Doubts are not uncommon at all. I spent years in doubt and wondering if I would be making a mistake transitioning. I even doubted myself for the first 6 months on HRT, but gradually as I realized I was becoming who I was inside, the doubts began to slowly lose their grip and fade into the background.
I remember someone told me that when we doubt ourselves, we're actually analyzing ourselves at a deeper level, and for us that are transgender, we really go deep into inspecting our internal selves. Some doubt is healthy. I would be concerned about someone jumping into transition without a single doubt as to what they were doing.