Thanks, Gaius. The thread you linked to is indeed very helpful, and I also didn't know about the Legamus Reader series, so I've ordered the Cicero and am looking forward to it.
Interestingly (to me, at least), once I gave up on Livy's preface and went to the actual history, things got much easier (still not easy, by any means, but not defeatingly difficult). I'm using a student edition with helpful notes. I wonder what it is in the preface that's different from the history--I suppose there's a lot more counterfactual stuff, and a lot more speculation and musing, whereas the history itself is very much about "this happened and then this happened" and so feels more straightforward.
In any case, thanks so much for your reply, which was just what I needed in the moment.