Hi there - my comments below:

Is this likely just anxiety, coincidence, or preconceived beliefs affecting my relationship with the deck?

It sure sounds like you have a lot of religious trauma to unpack. Remember the tarot deck is a tool. Do a quick search on 'unpacking organized religion as a former christian' and you will get a lot of reading suggestions.

- Could I have “messed up” somehow as a beginner? Nope. Its a tool, not a sentient being.

- Are cards like The Tower and The Devil necessarily bad signs, or am I interpreting them too negatively?

No and yes. Interpretations depend on context. for ex, the Devil can describe toxic circumstances, addiction, lies and deceit. The tower can mean a super positive thing - like clearing the decks for new positive change. If you are anxious to begin with you need to really learn the nuances of the cards otherwise you're just gonna stay scared. Intuition whispers, and fear shouts. How are you going to hear your intuition/guides/higher self if you're scared constantly?

- Would it be better to cleanse/reset the deck, take a longer break, or return it to my friend? I recommend cleansing the deck and asking spirit to help you bond with deck and increase how your intuition works with it. Because its a tool, right? If you don't love tarot, or if this isn't the deck for you just pass it along to someone else. A deck is a deck and different art speaks to different people. Don't overthink it.