Is the underlying issue that the op is anxious that the car is going to break down and this is causing him stress. Whereas partner is more relaxed- when it breaks we’ll either fix it or buy another sort of attitude. So it’s not a money issue.
If that is the case maybe try setting it out like that ie I’m worried it will break down and would sleep easier at night, focus at work better if we got a new one. Rather than it ‘might’ break down. It then becomes a double empathy problem, can see she put her self in your shoes to appreciate you are stressed about it.
Any car ‘might’ break down, even a newer one. So As a general rule the car you have is usually the cheapest option over a newer one unless someone catastrophic fails. That is unless you are RNP when people seem to give him cars for a fraction of the amount WBAC would give them even if the thing was a battered wreck!