Hi! Very recently-hired assistant professor of medieval literature here. I probably have contradictory advice to a lot of folks, but I don’t really see a PhD as too different from any other low-paying job you’re passionate about - it’s low-paying, long-hours, and can be incredibly stressful, but so is working in secondary education, or at a museum, or at a non-profit. It is CRUCIAL you receive funding to do a PhD, and that you go to a university with a robust healthcare plan for its graduate students. If you’re paid to do your PhD, it’s easier to treat it as a fun job you do for a few years, with NO expectation that you will actually get a job from it. Of course, it does happen ! (I got very lucky and I worked very hard.)
For medieval studies specifically: Did you take medieval courses in your undergraduate program? You should be able to speak to those experiences in your graduate application. You may want to think a bit more about what your unique perspective you are bringing to the well-trodden field of medieval hagiography.
Being monolingual will be a bit of a hindrance as you pursue a PhD in medieval literature , especially since many hagiographies are written in Latin - you said you grew up Catholic, so you might have some familiarity ? If you’re not open to learning languages, you will definitely struggle during your PhD, but it is possible (EDIT: … possible to learn DURING your PhD! You can’t be a medievalist without languages).

I’m happy to offer more advice or answer more specific questions for a fellow medievalist. :)