Harriet is clear-sighted and sensible right up to the point when she gives into being madly in love. Maybe she's less cautious about that the first time round, not having been burnt by experience. Or maybe Phil, unlike Peter, plays the physical card, snogs her senseless, and then only has to get over the proprieties of not wanting to marry. But you're right, he must have something to offer, and that to be enough that when Harriet does chuck him it isn't that she's glad to have an excuse to end a relationship that has palled on her, but she's genuinely devastated, even if some of that is feeling that she's got it all wrong. "Interesting self-destructiveness" doesn't sound implausible, what with his histrionic letters after they've separated.

a secondary reason why Harriet takes so long to admit to herself that she's in love with Peter is that it isn't the first time she's gone up like straw, and post-Phil she no longer trusts that feeling

Definitely! Possibly while trying to convince herself that she's not that sort of person at all...