I thought this guest post was great, but I wanted to add something that I realized after my ADD diagnosis at age 60 and 39 years of marriage.
After learning about the ADD time warp (which Melissa incredibly accurately refers to as now-not now), I realized that a huge proportion of my clashes with my wife over spending were at least partly the result of the now-not now time reality that I inhabit.
Non-ADD partners may have a hard time believing this, especially from a high-earning professional with post-graduate degrees, but I simply cannot connect what I spent this week to what I spent last week or what will need to be spent next week. The time horizon may be a little longer than a week, but it isn't much longer.
My realization of that hasn't magically fixed the issue, but awareness has been a significant help. If you feel like you ADD/ADHD partner doesn't "get it" about money issues, perhaps you should talk about how your partner experiences time.