I am also a christian and went through a phase similar to yourself. If not shorter and possibly less intense. A cloud would come over my head for a time and I worried that one day it would stay.
I think my fear of death stemmed from being part of a loving family for the first time in my life. This churned up emotions inside of me that I had never had before. The fear of being separated from people who loved me was new. It took some time and discussion for me to realise that these feelings were entirely normal. It became a matter of what I chose to do with these thoughts and feelings.
As the response above suggests you should speak to an expert. Hopefully they will help you break out of the rut you are getting yourself into. The common sense part of me recommends you focus on today, then tomorrow, then the day after. At 50 I focus on Jesus's teaching of not unnecessarily worrying about tomorrow. My prayer is often, "God give me the wisdom I need for today". This prayer follows years of trying to anticipate the future and being confused by much of the "be awesome" tat we read online.