This might be a stupid question, but what is "identity" in this context?

I ask because when this subject has come up I realize I don't seem to be able to think up theories about myself so easily

Run a few test theories and see what resonates. Are you the kind of person who goes to the gym every day? Are you the kind of person who works on their hobbies at night? Are you the kind of person who binges Netflix between dinner and bed? Are you the person who cooks dinner from scratch, or orders takeout at every chance? etc. I think of them a bit like stereotypes but applied to myself, and see if I think I fit.

It’s not supposed to be descriptive in this case, unless it’s a reflection or self-assessment. For goals, it’s supposed to be aspirational, like a vision you cultivate for yourself. You can meditate on that self-image, or identify other people with that trait and associate yourself with them, etc. The idea is that you will violate your sense of self by acting against how you expect yourself to be.

I'll look into it. I think I acquired frugality from reading a few books that made me frugal.

I'd like other traits too.

First time I’m reading about a character trait walking straight off the page into the reader’s life..