Maybe this self-doubt means you are ready to take a different path in life (at least a different inner path). Maybe the old concepts you have of yourself don't fit who you know you are now and you are giving them up, while you are looking to build a new self-concept. Sometime you just have a feeling that you are not what you thought you were (and even what others think you are), but you have no clear picture of who you, in fact, might be. So you just have to drift along in a rather amorphous space for a time, as the picture comes into focus. But, being open to not having all the answers can bring some very interesting new possibilities into your life. Although it feels like forever, these transition periods are often really quite brief. Everyone who goes through personal growth and development goes through these phases of discarding the old, outmoded concepts of the self and discovering/crafting new concepts that jive with the new you that you are discovering. I think part of that is healthy introspection about who we are and what we are doing in life. If that is what is happening to you, there are life coaches, spiritual advisers, professional mentors, etc. that can help you with this. On the other hand, if this is just a funk you have fallen into, it will pass in a day or two and you will feel like your old self again very soon.