How is your book going on? As I know, guys with mental health problems are able to release really distinctive things. Just turn your "minus" into "big plus". I think all of those fears are from a lack of self-confidence. As a suggestion, you can start by working out. Such sites as
fitbodybuzz.com contain all the necessary info about training. Strong men have nothing to fear
By the way, what exactly your fantasy novel will be about? You mentioned dragons, knights, spellblades, and the protection of the Cloak of Mylencia. Cloak of Mylencia is a "sacred relic" of your fantasy world?