If I had to consider a cohabitation agreement, or keeping a "stash" for the possibility of my significant other taking me to the cleaners.....I think I'd just be staying well away from any situation like that.
Besides, cohabitation agreements are only as good as the judge say's they are.....like everything else, it comes down to who has the best lawyer.
To the subject at hand though......it's a bit of a sticky wicket. You're "hitched", papers and all.....she's not good with money and doesn't want you advising her on how to handle it. This does not bode well for a stable future.
This may sound crazy, but I've observed that a lot of women come to resent the help that their "other" provides, even though they know they need it. It seems to trigger some defense mechanism in them and they end up actually blaming their partner.....or, they just don't want to take responsibility for their mistakes.
Since you're asking.....on a cast bullet site of all places, but still, you did ask. I don't think you're overreacting......but I also think you're enabling her to continue in her bad habits. She needs to find her own way, very likely via the hard way.
That "stash" might come in handy.