I think what you are experiencing is normal. You're about to turn 40. It makes us evaluate our lives. We ask ourselves purpose filled existential questions. It's a self-check that doesn't necessarily happen when we turn 30 but each decade after that we begin to reflect. I think for writers many of us think about our purpose a lot because this is why we write to touch other people's lives in some way. Be easy on yourself and be open which it sounds like you already are.