@Lady Wolfshead : It sounds as if you've found a comfort zone for your uncertainty, and made or arrived at a decision with which you can accept. If not, or you start to again have regrets or doubts; just go back & look at some of your own quotes-
"My husband thinks I shouldn't go. And I have to admit that even though she is a pretty good friend, she would never make that trip for me despite owning her own car." // "So I discussed my situation regarding my friend's birthday with another friend, who said basically don't give it another thought."
Only thing I might suggest, should it still be even a mini-disturbance, is that to look at it logically and/or rationally. That is to say - with your brain - & not your heart. Also, if you sort of take the advice you so often offer others in sometimes more dire & severe circumstances, view the situation, or the circumstance from the outside (perspective/point-of-view). And analyze it that way. Remove your-self from the equation. And just look at the facts.
It seems to me pretty reasonable to have arrived at your said conclusion, then. And to not have any more reservations. Unless, of course, if her or she (your friend) who is troubling you with any kind of pressure, blame/shame, guilt or otherwise perceived . . . basically, "You're (really) letting me down, you know?" / "I was counting on you!" If it's implied, as if you'd made a promise, and are now unrelinquishing it (has she this power w/o saying a single word?) - then given the nuts & bolts of the deal itself (such as the realities of the car, for instance) to me, make it extraordinarily easy to justify & defend on a practical level.
Heck, even with a vehicle, I don't know that I'd love driving to an unknown city, for the first time, by my self (even erase the drive-time); just navigating the new city like that, under those conditions. Well, again to me anyway, that's a recipe for disaster. Or if such should happen, then I'm almost at fault for tempting the Fate's my-self! But I've got a funny way of thinking sometimes... Respectfully, Yours.