Why? Our assurance of eternal life isn’t in our works, but in God’s commitment and competence in empowering and leading us to be righteous. I certainly didn’t mean to be saying that we earn our eternal life once we attain enough righteousness to be justified. (Nor do we earn it by faith instead of works, which only makes faith itself a work.)
Of course the term was a contemporary legal term, but God’s judgment isn’t ever a merely legal judgment (maybe never a legal judgment at all, except in passing). God judges truly and truly judges what actually is true. God’s judgments aren’t a useful legal fiction like human legal justification at the time (and even that wasn’t only legal in basis: the core concept still goes back to a judge correctly judging a person to be righteous.)
And not your trust in God Himself, personally? That can be hard to do, admittedly, but I think it’s better than trusting a legal formality, especially what is only a legal fiction (however useful it might be). But God is faithful to us regardless of our difficulties in trusting Him personally. And being a perfectly just judge, God relates to us taking all our difficulties fairly into account. (Which is why God expects more from those who have more advantages.)
George MacDonald, C. S. Lewis’ teacher, has several interesting and helpful considerations of the topic in his Unspoken Sermon series (including what amounts to volume 4, The Hope of the Gospel), if you can find those.