Your divorce settlement was generous and, after 25 years of relationship, I have no doubt that she worked hard helping to raise your kids. You would, of course, know more about that than I. Kudos for doing the right thing when so many divorcing spouses can’t see beyond their own resentments.
Your divorce settlement also gave your children peace of mind, which is difficult to put a cost on. Whatever the nature of their relationship, I assume they love her very much. It’s hard to untangle oneself from the emotional ties as well as the financial ties, but you seem ready.
Once you make a settlement in a divorce, it’s difficult to revisit it, especially if there were no conditions set at the time. If a divorce settlement doesn’t stipulate that the lower-earning partner had to find a full- or part-time job within a certain period, it’s frequently difficult to change.
Including “modifiable maintenance” in your divorce settlement was a smart move on your part and, given your wish to reduce it, is a good start. That, and the fact that the rest of the settlement was generous, will bode well for you if this gets to court.
Your wife may argue, with reason, that she was married to from the beginning of your career throughout the peak earning years, and raising children and managing a home is a full-time job, and a significant one, in itself.